US writer from South interrupting fat cat (7)
I believe the answer is:
leopard
'cat' is the definition.
(leopard is a kind of cat)
'us writer from south interrupting fat' is the wordplay.
'us writer' becomes 'poe' (writer Edgar Allan Poe).
'from south' shows that the letters should be reversed in order.
'interrupting' is an insertion indicator.
'fat' becomes 'lard' (lard is a kind of fat).
'poe' reversed gives 'eop'.
'eop' going within 'lard' is 'LEOPARD'.
(Other definitions for leopard that I've seen before include "Animal -- per load (anag)" , "Panther" , "Spotted big cat" , "dangerous when free" , "Spotted beast" .)