It's certainly not out of action (6)
I believe the answer is:
indeed
'it's certainly' is the definition.
(both can be used to answer in the affirmative)
'not out of action' is the wordplay.
'not out of' 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" , "To be sure" , "Word expressing surprise, disbelief, etc" , "Certainty" , "In reality" .)