Set by the boy to trap one (4)
I believe the answer is:
laid
'set' is the definition.
(synonyms)
'the boy to trap one' is the wordplay.
'the boy' becomes 'lad' (lad is a kind of boy).
'to trap' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'lad' going around 'i' is 'LAID'.
'by' is the link.
(Other definitions for laid that I've seen before include "Produced (an egg)" , "(Of dining table) set" , "Placed horizontally on a surface" , "settled" , "Put down like dial" .)