"How you doin'?" (3)
I believe the answer is:
sup
'how you doin'?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sup that I've seen before include "Drink; eat evening meal" , "Drink -- eat (with the devil?)" , "Drink; take a late meal" , "Take food" , "Drink; take an evening meal" .)