Popular act for sure (6)
I believe the answer is:
indeed
'sure' is the definition.
(I've seen this before)
'popular act' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'act' becomes 'deed' (deed is a kind of act).
'in'+'deed'='INDEED'
'for' acts as a link.
(Other definitions for indeed that I've seen before include "Yes, truthfully" , "still" , "In truth, in fact" , "beyond doubt" , "Word expressing surprise, disbelief, etc" .)