Hat produced from behind boy's back (5)
I believe the answer is:
derby
'hat' is the definition.
(I know that derby is a type of hat)
'from behind boy's back' is the wordplay.
'from' becomes 'by' (by can mean from a particular person).
'behind' means one lot of letters go next to another.
'boy' becomes 'red' ().
'back' shows that the letters should be reversed in order.
'red' in reverse letter order is 'der'.
'by' put after 'der' is 'DERBY'.
'produced' is the link.
(Other definitions for derby that I've seen before include "Hat, race and local match" , "American bowler" , "close match?" , "Commotion" , "Sports match between local rival teams" .)