Old boxer's manager gained zero (6)
I believe the answer is:
cooper
'zero' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'old boxer's manager gained' is the wordplay.
I cannot really understand how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found within the answer.
'manager' could be 'coper' (someone who manages or copes) and 'coper' is found in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cooper that I've seen before include "supplier of stiffeners?" , "He makes and repairs barrels" , "Maker of barrels" , "Craftsman" , "Barrel-maker" .)