I don't think it's unreasonable for Clinton to be mad. This is a guy who needed a lot of time to psyche himself up for his first date, and still freaked out as soon as he actually met up with her; he likes to have time to mentally prepare himself for social interactions, at least those of the dating variety (see 2933). Even after he got a kiss on the cheek after their first date, he was still scared to ask her out again, and is visibly nervous when Claire arranges for them to meet up by surprise (see 2997). Even though that interaction goes well, he is still scared to make another move. Clearly, he is an extremely introverted person who is uncomfortable with romantic interactions and impromptu meetups.
Now, Claire has tricked him into another surprise meeting, and instead of this encounter having the appearance of an innocent meet-up, Claire says to him in front of Emily that the purpose of this meeting is for him to ask her out again. This is a nightmare for introverts like Clinton. A minute ago, he had been spending the day with his sister; while they may fight from time to time, he's known Claire his whole life, and is probably very comfortable around her socially. Now he's been kicked out of his comfort zone with zero warning. He had no chance to mentally prepare to interact with somebody who makes him extremely nervous, and he can't even play this off as a meeting between friends because of Claire's comments. He has to ask her out, which he hadn't wanted to do over the last few months for two reasons:
1. He isn't sure if he wants to go steady with Emily
2. He is afraid of rejection
Therefore, he is very stressed out & uncomfortable when Claire puts him on the spot. Then, he gets shut down, in front of his sister to boot. I know Emily tried to let him down gently, but for someone as insecure as Clinton, there is no such thing as a breakup which feels good. So Clinton is currently angry at having been set up, stressed over having to interact with Emily with no warning, and embarrassed at being rejected. On top of that, he learns that Claire had no idea whether things were going to work out, and decided to force the issue anyway.
So Clinton feels like Claire threw him into a situation way outside his comfort zone without asking, and it went poorly on top of that. Is Clinton being a jerk, and will he regret what he said later? Yes. But right now he's hurt and lashing out. I don't want to defend his actions, but as an extremely introverted person I can totally understand how Clinton is feeling right now.