Boy accepts one put down (4)
I believe the answer is:
laid
'put down' is the definition.
(to lay something is to put it down somewhere)
'boy accepts one' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'accepts' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'lad' enclosing 'i' is 'LAID'.
(Other definitions for laid that I've seen before include "Produced (an egg)" , "Doze" , "settled" , "Placed horizontally on a surface" , "Prepared (a trap)" .)