Not the whole catch is thrown back (4)
I believe the answer is:
part
'not the whole' is the definition.
I can't judge whether this definition defines the answer.
'catch is thrown back' is the wordplay.
'catch' becomes 'trap' (trapping is a kind of catching).
'is thrown back' says the letters should be written in reverse.
'trap' in reverse letter order 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" .)