Successful mouse-catcher presumably does the business better than most (3,3)
I believe the answer is:
fat cat
'most' is the definition.
Both the definition and answer are singular nouns.
Maybe there's a link between them I don't understand?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for fat cat that I've seen before include "Wealthy and powerful person (informal)" , "Moneybags as an obese feline" , "pampered pet" , "Banking executive maybe" , "Rich person or obese feline" .)