Trendy act? Absolutely (6)
I believe the answer is:
indeed
'absolutely' is the definition.
(I've seen this before)
'trendy act?' is the wordplay.
'trendy' becomes 'in'.
'act?' becomes 'deed' (deed is a kind of act).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "Really - what a surprise!" , "In reality" , "Truthfully, really" , "still" , "''A friend in need is a friend . . . . . .''" .)