During action, to be sure (6)
I believe the answer is:
indeed
'sure' is the definition.
(I've seen this before)
'during action' is the wordplay.
'during' becomes 'in' (e.g. during February means in February).
'action' becomes 'deed' (deed is a kind of action).
'in'+'deed'='INDEED'
'to be' acts as a link.
(Other definitions for indeed that I've seen before include "Expression of interest, incredulity or contempt" , "Really, actually" , "Surely" , "Certainty" , "In fact, really" .)