Macabre writer backed into fat cat (7)
I believe the answer is:
leopard
'cat' is the definition.
(I know that leopard is a type of cat)
'macabre writer backed into fat' is the wordplay.
'macabre writer' becomes 'poe' (I've seen this in another clue).
'backed' says the letters should be written in reverse.
'into' means one lot of letters goes inside another.
'fat' becomes 'lard' (lard is a kind of fat).
'poe' written backwards gives 'eop'.
'eop' placed inside 'lard' is 'LEOPARD'.
(Other definitions for leopard that I've seen before include "Spotted feline" , "dangerous when free" , "Spotted beast" , "Large spotted cat" , "The cat that does not change its spots" .)