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 cannot understand how they can define each other.
'old boxer's manager gained' is the wordplay.
I cannot really see how this works, but
'old' could be 'o' (abbreviation) and 'o' is found within the answer.
'manager' could be 'coper' (someone who manages or copes) and 'coper' is found within 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" .)