Trendy act certainly (6)
I believe the answer is:
indeed
'certainly' is the definition.
(both can be used to answer in the affirmative)
'trendy act' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'act' becomes 'deed' (deed is a kind of act).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "Really, without doubt" , "Really - truly" , "In reality" , "Unquestionably" , "Very much so" .)