"What's happenin'?" (3)
I believe the answer is:
sup
'what's happenin'?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sup that I've seen before include "Drink; take a late meal" , "A mouthful of liquid" , "Take food" , "Eat a late meal; sip" , "Drink; eat evening meal" .)