Writer turned into fat cat (7)
I believe the answer is:
leopard
'cat' is the definition.
(leopard is a kind of cat)
'writer turned into fat' is the wordplay.
'writer' becomes 'poe' (Edgar Allan Poe).
'turned' shows that the letters should be reversed in order.
'into' is an insertion indicator.
'fat' becomes 'lard' (lard is a kind of fat).
'poe' backwards is 'eop'.
'eop' going into 'lard' is 'LEOPARD'.
(Other definitions for leopard that I've seen before include "Spotted beast" , "Powerful cat" , "Panther" , "The cat that does not change its spots" , "Large spotted cat" .)