Based upon a particular gif seen from s8 filming. If you know of the "thank you for him" bit, then you've got it.
She's looking at him – staring with squinted eyes, scrutinizing with a twitching upper lip, studying close but keeping her distance – and it's making him squirm.
The Doctor inhales as she steps to the right and seems to take special interest in his ears – oh. It's not the ears, no way. Those were normal. It's the jowls, isn't it? The added wrinkles in his neck, the looseness of the skin that curves around his jaw. He's thinner now (somehow) and taller too, but those attributes just don't matter when the body they belong to are physically older. He can feel it.
He can feel her eyes boring into the noticeably different features from before, see the calculations running in her mind that are drawing comparisons. Maybe her expression remains neutral in study, and maybe he's just imagining it – but no. The rolling acid in his gut tells him otherwise. It tells him the truth because his anxiety-rattled brain is trying so hard to calm him down, it has lost sight of what has occurred:
He changed.
He, the Doctor, changed, and had accidentally left her behind. She looks just a bit different now, but very much so the same...was she always this small? Wasn't her nose cuter? Wait, no – noses didn't change, not on humans, not naturally at least. Was her nose always cute? Did he...always find it cute? Were those tiny crinkles at the corners of her eyes always so endearing, and were her teeth always shaped in such ways, and was her chin just as defiant as it had always been?
Was she different too?
No...
He was the different one, and he could feel the distance between them growing as she takes a step back to observe him from the front. His spine stiffens as she then ducks to the left, sending another searing glance over all the imperfections he can possibly think of – imperfections because they are not what she wants to see.
I'm not different, I haven't changed he wants to say to her. He wants to do something, to reassure her, to let her know...but nothing is cooperating. His body is frozen, locked down by her wandering eyes that flicker back and forth over his face, trailing down his neck and judging – judging, yes, he knows it. He just...knows it.
Because he's not the same to her.
The bile rises to his throat as he finally exhales slightly, lowering his brow and allowing his thoughts to nestle into defeat. It's okay they whisper to him. You never stood a chance anyway, may as well not get in too deep.
She is awfully clever.
And pretty.
And too good for you.
The Doctor's spine sinks as he closes his eyes, his lungs compressing, his hearts slowing. He has to get past this, he has to move on – what did he really think was going to happen? Change is difficult, sometimes impossible, to handle, and for what he had meant to her...he wasn't so thick, he knew; and even if he didn't know before he knew now, the way her eyes ached as they longed for features that she would never see again – for the chin, the thin eyebrows, the triangular nose that was decidedly not hooked.
She would never look at him the same way again. He knew.
"Thank you," Clara's voice breaks quietly into his thoughts. The Doctor feels something rise in his chest, something small but swelling but just too bright. His eyes open in time to see her rise on her toes and loop her arms around his neck, one elbow resting on his shoulder as her other hand grazes his back, mobile in the way. He freezes completely, his arms halfway up but fingers clenched, mind racing in a frenzied panic as it questions what to do.
What's going on.
How is this happening.
This isn't happening.
"Thank you...for him," she says quietly. His eyes widen as his hearts skip, his breath catching, his stomach dropping. But she curls her fingers tighter into him, and he swears he can feel her nails pricking through as she burrows her face deeper into his collar.
"Thank you," she breathes, her words barely emerging from being spoken into his jacket, "for you."
His hearts resume beating as something washes over him...something bright, something warm, something that sends a shudder through his chest and brings a sting to his eyes. His hands don't draw closer, and he his arms still won't cooperate, but he leans his neck forward, nestling his chin into her shoulder, gently resting his cheek against her hair. She is small and quiet but all together comfortable, and he feels his skin flush as she pats his back.
This is my Clara.