"Yeah, why not?" (4)
I believe the answer is:
sure
I believe this clue is a double definition.
'yeah' is the first definition.
(thesaurus)
'why not?' is the second definition.
(I've seen this in another clue)
(Other definitions for sure that I've seen before include "Undoubtedly true" , "The old sewer" , "Safe" , "Certain to be a ruse" , "Definite" .)