Popular achievement for sure (6)
I believe the answer is:
indeed
'sure' is the definition.
(I've seen this before)
'popular achievement' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'achievement' becomes 'deed' (deed is a kind of achievement).
'in'+'deed'='INDEED'
'for' acts as a link.
(Other definitions for indeed that I've seen before include "Really, without doubt" , "Truthfully, really" , "In fact, really" , "Certainty" , "Really - truly" .)