In fact in the act (6)
I believe the answer is:
indeed
'in fact' is the definition.
'in the act' is the wordplay.
'the act' becomes 'deed' (deed is a kind of act**).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "Certainly, quite so" , "Surely" , "Really, without doubt" , "To be sure" , "Word expressing surprise, disbelief, etc" .)