Trendy action for sure (6)
I believe the answer is:
indeed
'sure' is the definition.
(I've seen this before)
'trendy action' is the wordplay.
'trendy' becomes 'in'.
'action' becomes 'deed' (deed is a kind of action**).
'in'+'deed'='INDEED'
'for' acts as a link.
(Other definitions for indeed that I've seen before include "Yes, truthfully" , "Actually -- denied (anag)" , "Certainly, quite so" , "Very much so" , "In truth, in fact" .)