Catch mate briefly on the way up (6)
I believe the answer is:
'catch' is the definition.
(to entrap is to catch with a trap)
'mate briefly on the way up' is the wordplay.
'mate' becomes 'partner' (romantic or sexual partner).
'briefly' means to remove the last letter.
'on the way up' is a reversal indicator.
'partner' with its last letter taken off is 'partne'.
'partne' back-to-front is 'ENTRAP'.
(Other definitions for entrap that I've seen before include "Something to do with gin" , "Snare, trick" , "Capture" , "Lure into danger" , "Catch in a snare" .)