This holds fodder — not quite enough to make nag obese! (7)
I believe the answer is:
nosebag
'this holds fodder not quite enough to make nag obese' is the definition.
The definition suggests a singular noun which matches the answer.
'this holds fodder not' is the wordplay.
I cannot really understand how this works, but
'not' could be 'no' (negative) and 'no' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for nosebag that I've seen before include "Container for a horse's dinner" , "Horse's fodder container" , "Portable container for horse's food" , "Feeder strapped on horse" , "Horse's fodder pouch" .)