I just happen to know who click clacking, you mismatching

from A$AP Rocky (Ft. ScHoolboy Q) – Brand New Guy Lyrics on Rap Genius

Meaning

Though he’s not in a gang, he still knows his gang, unlike many people that claim to to be one set, yet not know the rules, or can follow them well.

As he has shown previously, he knows gang members on both sides very well.

“I just happen to know who click clackin"
Because of these connections he knows who shot whoever. "Click clackin” is onomonopoetia for gunshots.

“You mismatchin"
Youve got the wrong guy.

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