English musical work cornered by fat cat (7)
I believe the answer is:
leopard
'cat' is the definition.
(leopard is a kind of cat)
'english musical work cornered by fat' is the wordplay.
'english' becomes 'e' (abbreviation).
'musical work' becomes 'op' (abbreviation for opus).
'cornered by' indicates putting letters inside (I've seen this in other clues).
'fat' becomes 'lard' (lard is a kind of fat).
'e'+'op'='eop'
'eop' placed inside 'lard' is 'LEOPARD'.
(Other definitions for leopard that I've seen before include "Spotted big cat" , "Spotted beast" , "Wild cat" , "dangerous when free" , "Big cat with spots" .)