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 backwards.
'part' reversed gives 'TRAP'.
(Other definitions for trap that I've seen before include "Part (anag.)" , "mug" , "Deceive" , "Light horse-drawn carriage" , "Cakehole (slang)" .)