Popular action? Certainly! (6)
I believe the answer is:
indeed
'certainly' is the definition.
(thesaurus)
'popular action?' is the wordplay.
'popular' becomes 'in'.
'action?' becomes 'deed' (deed is a kind of action**).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "Really; certainly" , "Really, without doubt" , "Really - truly" , "Absolutely" , "Really - what a surprise!" .)