Popular exploit for sure (6)
I believe the answer is:
indeed
'sure' is the definition.
(I've seen this before)
'popular exploit' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'exploit' becomes 'deed' (both can mean a feat or escapade).
'in'+'deed'='INDEED'
'for' is the link.
(Other definitions for indeed that I've seen before include "even" , "still" , "Really - truly" , "In fact, really" , "Really; certainly" .)