Catch constituent up (4)
I believe the answer is:
trap
'catch' is the definition.
(trapping is a kind of catching)
'constituent up' is the wordplay.
'constituent' becomes 'part' (I've seen this before).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'part' written backwards gives 'TRAP'.
(Other definitions for trap that I've seen before include "Kisser" , "get in net" , "Baited device" , "It commonly has a role in speaking" , "Device to catch prey" .)