A close synonym for "fit" here would be "match". Here's an old English joke: A man goes to his doctor and says to him "Doctor, doctor! I feel like a pair of curtains." The doctor replies : "Pull yourself together." The joke here is that one "pulls together" curtains to close them. "Pull yourself together" is also a common expression which means that you should stop worrying about something and start to be sensible - we can imagine a strict doctor saying this. The joke manipulates words in a clever way. "Semantic" means relating to the usage of words. And so a "clever semantic fit" expresses the idea that one set of words in a joke is used in a clever way so that they mean two contrasting things. Hopefully, the contrast will be absurd and make you laugh.

I think that it is referring to when you realise something important... And ''fit'' is there as an adjetive for clever.