To be sure within the act (6)
I believe the answer is:
indeed
'to be sure' is the definition.
(I've seen this before)
'within the act' is the wordplay.
'within' becomes 'in'.
'the act' becomes 'deed' (deed is a kind of act).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "still" , "In fact, really" , "Really, without doubt" , "Surely" , "Actually -- denied (anag)" .)