Source of energy — a cat? (4)
I believe the answer is:
atom
'source of energy' is the definition.
(I've seen this in another clue)
'a cat?' is the wordplay.
'cat?' becomes 'tom' (a tom is a male cat).
'a'+'tom'='ATOM'
(Other definitions for atom that I've seen before include "A mighty physical particle or bomb" , "Small amount of matter" , "Not much substance" , "A tiny powerful particle" , "Piece of matter in moat" .)