It's in action certainly (6)

I believe the answer is:

indeed

'certainly' is the definition.
(both can be used to answer in the affirmative)

'it's 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" , "Actually -- denied (anag)" , "Really, actually" , "Yes, truthfully" .)

I've seen this clue in The Globe and Mail.

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