Writer eaten up by fat cat (7)
I believe the answer is:
leopard
'writer' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'eaten up by fat cat' is the wordplay.
'eaten up by fat' becomes 'leo' (I can't explain this - if you can you should believe this answer much more).
'cat' becomes 'pard' (archaic term for leopard).
'leo'+'pard'='LEOPARD'
Can you help me to learn more?
(Other definitions for leopard that I've seen before include "Big cat with spots" , "Spotted animal" , "The cat that does not change its spots" , "Animal -- per load (anag)" , "Powerful cat" .)