Man taking part, showing enthusiasm (4)
I believe the answer is:
gent
'man' is the definition.
(I know that gent is a type of gentleman)
'part showing enthusiasm' is the wordplay.
'part' indicates the answer is hidden within the clue.
'GENT' is hidden within 'showing enthusiasm'.
'taking' acts as a link.
(Other definitions for gent that I've seen before include "Auctioneer's tool" , "bloke" , "Chap, fellow" , "cove" , "Informal and short man" .)