US writer turned into fat cat (7)
I believe the answer is:
leopard
'cat' is the definition.
(leopard is a kind of cat)
'us writer turned into fat' is the wordplay.
'us writer' becomes 'poe' (I've seen this before).
'turned' is a reversal indicator.
'into' is an insertion indicator.
'fat' becomes 'lard' (I've seen this before).
'poe' reversed gives 'eop'.
'eop' going inside 'lard' is 'LEOPARD'.
(Other definitions for leopard that I've seen before include "Spotted big cat" , "dangerous when free" , "Animal -- per load (anag)" , "Wild cat" , "Spotted feline" .)