Writer upset, taken in by fat cat (7)
I believe the answer is:
leopard
'cat' is the definition.
(leopard is a kind of cat)
'writer upset taken in by fat' is the wordplay.
'writer' becomes 'poe' (Edgar Allan Poe).
'upset' shows that the letters should be reversed in order.
'taken in by' indicates putting letters inside.
'fat' becomes 'lard' (lard is a kind of fat).
'poe' backwards is 'eop'.
'eop' put into 'lard' is 'LEOPARD'.
(Other definitions for leopard that I've seen before include "Panther" , "Spotted animal" , "dangerous when free" , "The cat that does not change its spots" , "Powerful cat" .)