Certainly popular act (6)
I believe the answer is:
indeed
'certainly' is the definition.
(thesaurus)
'popular act' is the wordplay.
'popular' becomes 'in'.
'act' becomes 'deed' (deed is a kind of act**).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "Certainly, quite so" , "even" , "Expression of interest, incredulity or contempt" , "Really; certainly" , "still" .)