Popular action, no doubt (6)
I believe the answer is:
indeed
'no doubt' is the definition.
(both can be used to express agreement)
'popular action' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'action' becomes 'deed' (deed is a kind of action).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "Really; certainly" , "Truthfully, really" , "Surely" , "Without question" , "Yes, truthfully" .)