Not the whole catch is thrown back (4)
I believe the answer is:
part
'not the whole' is the definition.
I can't tell whether this defines the answer.
'catch is thrown back' is the wordplay.
'catch' becomes 'trap' (trapping is a kind of catching).
'is thrown back' shows that the letters should be reversed in order.
'trap' back-to-front is 'PART'.
Can you help me to learn more?
(Other definitions for part that I've seen before include "Member" , "Bit - leave" , "Portion of a whole" , "Component; divide" , "character on > stage" .)