Certainly popular feat (6)
I believe the answer is:
indeed
'certainly' is the definition.
(thesaurus)
'popular feat' is the wordplay.
'popular' becomes 'in'.
'feat' becomes 'deed' ('deed' can be a synonym of 'feat'**).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "Surely" , "Expression of interest, incredulity or contempt" , "Truly or admittedly" , "Really - what a surprise!" , "Really, actually" .)