Mafia boss ultimately shown to be chicken (5)
I believe the answer is:
capon
'be chicken' is the definition.
(I've seen this in another clue)
'mafia boss ultimately shown' is the wordplay.
'mafia boss' becomes 'capo' (head of a crime syndicate).
'ultimately' says to take the final letters.
The final letter of 'shown' is 'n'.
'capo'+'n'='CAPON'
'to' acts as a link.
(Other definitions for capon that I've seen before include "Male chicken for eating" , "fowl fattened for pot" , "Castrated rooster fattened for eating" , "Castrated cock fowl" , "A tasty bird" .)