One in boat crew heard birds (3)
I believe the answer is:
cox
'one in boat crew' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'heard birds' is the wordplay.
'heard' shows a homophone (sound like).
'birds' becomes 'cocks' (cock is a kind of bird).
'cocks' sounds like 'COX'.
Can you help me to learn more?
(Other definitions for cox that I've seen before include "Helmsman" , "Steerer of oarsmen in rowing boat" , "Passenger vehicle" , "He's in charge of a racing boat and its rowers" , "Member of rowing crew" .)