Boy’s letters returned (4)
I believe the answer is:
liam
'boy's' is the definition.
(I've seen this before)
'letters returned' is the wordplay.
'letters' becomes 'mail' (I've seen this before).
'returned' is a reversal indicator.
'mail' back-to-front is 'LIAM'.
(Other definitions for liam that I've seen before include "-- Neeson (actor)" , "Irishman" , "Boy" .)