Certainly in action (6)
I believe the answer is:
indeed
'certainly' is the definition.
(both can be used to answer in the affirmative)
'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 "Certainly, quite so" , "Really - truly" , "Word expressing surprise, disbelief, etc" , "Very much so" , "Actually -- denied (anag)" .)