Energy and work embraced by fat cat (7)
I believe the answer is:
leopard
'cat' is the definition.
(leopard is a kind of cat)
'energy and work embraced by fat' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'and' says to put letters next to each other.
'work' becomes 'op' (abbreviation for opus).
'embraced by' is an insertion indicator.
'fat' becomes 'lard' (I've seen this before).
'e'+'op'='eop'
'eop' put inside 'lard' is 'LEOPARD'.
(Other definitions for leopard that I've seen before include "Panther" , "Large spotted cat" , "The cat that does not change its spots" , "Spotted big cat" , "Animal that doesn't change its spots" .)