Catch some in retreat (4)
I believe the answer is:
trap
'catch' is the definition.
(I know that trap is a more specific form of the action catch)
'some in retreat' is the wordplay.
'some' becomes 'part' (only part of something).
'in retreat' shows that the letters should be reversed in order.
'part' back-to-front is 'TRAP'.
(Other definitions for trap that I've seen before include "Control ball" , "Trick" , "Hook" , "Snare, ruse" , "Ambush" .)