Get one in a trap (4)
I believe the answer is:
gain
'get' is the definition.
(gaining is a kind of getting)
'one in a trap' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'in' is an insertion indicator.
'a trap' becomes 'gin' (gin trap is a kind of animal trap).
'a' going into 'gin' is 'GAIN'.
(Other definitions for gain that I've seen before include "Advance" , "Obtain to one's advantage" , "Benefit; increase in value" , "Obtain, secure" , "reward" .)