A/N: Based on a scene in S01E04. Billy's POV.
When Mr Reader says he's going to be a father, Billy actually laughs, says, "Who's the—" before it hits him. Clive just looks at him, his lips quirking slightly, as if Billy should have known all along who the father of Martha's baby was. Billy prides himself on knowing everything that happens in his barristers' lives, has known both Miss Costello and Mr Reader since they were barely more than kids, but this blindsides him. He knows that those two have a tempestuous relationship, that Clive probably fancies her, but he never thought Martha would reciprocate.
Only a couple of weeks ago he'd been laughing and whooping with the other clerks over Reader's antics. He'd scored again. This is different though, this isn't some pupil or solicitor, this is Martha Costello. The most beautiful, principled woman Billy has ever known.
It's not even that Billy wants to sleep with Martha himself, he's not in love with her in that way. He just can't stand the thought of Reader with his grubby hands all over her.
He lets Clive waffle on for a bit, about how impending fatherhood has changed him, bullshit upon bullshit, Billy's rage building all the while. Finally, he asks Clive what he wants. Once they've come to an agreement, Billy waits for Clive to lower his guard, then grabs him by the injured knee and twists.