Really fashionable act (6)
I believe the answer is:
indeed
'really' is the definition.
('indeed' can be a synonym of 'really')
'fashionable act' is the wordplay.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'act' becomes 'deed' (deed is a kind of act**).
'in'+'deed'='INDEED'
(Other definitions for indeed that I've seen before include "Really - truly" , "Unquestionably" , "Expression of interest, incredulity or contempt" , "still" , "Yes, truthfully" .)