Complete basic food, it’s said, for top-quality horse (12)
I believe the answer is:
thoroughbred
'top-quality horse' is the definition.
(eg a thoroughbred racehorse)
'complete basic food it's said' is the wordplay.
'complete' becomes 'thorough' (similar in meaning).
'basic food' becomes 'bread' (I've seen this before).
'it's said' indicates a 'sounds like' (homophone) clue.
'bread' is a homophone of 'bred'.
'thorough'+'bred'='THOROUGHBRED'
'for' is the link.
(Other definitions for thoroughbred that I've seen before include "No mongrel" , "full-blooded" , "A horse of pure lineage" , "Of outstanding quality" , "Animal of pure blood" .)