Catch some retreating (4)
I believe the answer is:
trap
'catch' is the definition.
(trapping is a kind of catching)
'some retreating' is the wordplay.
'some' becomes 'part' (only part of something).
'retreating' says the letters should be written in reverse.
'part' back-to-front is 'TRAP'.
(Other definitions for trap that I've seen before include "Part (anag.)" , "mug" , "Deceive" , "Light horse-drawn carriage" , "Cakehole (slang)" .)