The most that we can say at this point is that we don't know why Clinton was so upset about the way everything went when he spoke to Claire.
Personally, I'm not entirely in favour of the 'second thoughts' theory. Yes, Emily likes his spontaneity ( which, I argue, was always there, just waiting for the right stimulus to manifest itself - specifically, a pretty girl in a sundress). However, I'm not sure that Clinton can survive much more spontaneity. That said, he doesn't strike me as the pessimistic introspective sort like Marten. He may regret but I have a feeling that he'll at least try to be positive.
Additionally, there was something about Clinton's dishevelled state when Claire called that makes me think that he has one remaining misadventure awaiting him. Not Emily's fault but, nonetheless, rounding off the evening in fine style. I'm thinking something ridiculous like Punchy Bro throwing his keys in someone's yard pool for getting him tossed out of the bar (no assault - he decided that Elliot was right in that matter)!