Cockerel made man nervous at first (5)
'cockerel' is the definition.
'made man nervous at first' is the wordplay.
'made man' becomes 'capo' (type of mafia member).
'at first' suggests taking the first letters.
The first letter of 'nervous' is 'n'.
(Other definitions for capon that I've seen before include "Emasculated fowl", "Male chicken for eating", "Castrated rooster fattened for eating", "Castrated cock fattened for eating", "Male fowl fattened for eating".)