In action? Certainly (6)
I believe the answer is:
indeed
'certainly' is the definition.
(thesaurus)
'in action?' is the wordplay.
'action?' becomes 'deed' (deed is a kind of action**).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "Really, actually" , "still" , "Actually -- denied (anag)" , "Really - truly" , "Very much so" .)