Catch some up (4)
I believe the answer is:
trap
'catch' is the definition.
(trapping is a kind of catching)
'some up' is the wordplay.
'some' becomes 'part' (only part of something).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'part' backwards is 'TRAP'.
(Other definitions for trap that I've seen before include "Gin - hidden danger" , "get in net" , "Vehicle" , "website?" , "Deceive" .)