Really part of the act (6)
I believe the answer is:
indeed
'really' is the definition.
(synonyms)
'part of the act' is the wordplay.
'part of' becomes 'in' (being in something means being part of it).
'the act' becomes 'deed' (deed is a kind of act).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "Really, without doubt" , "Without question" , "still" , "Really; certainly" , "beyond doubt" .)