A cat scrap (4)
I believe the answer is:
atom
'scrap' is the definition.
(I've seen this before)
'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 "Tiny particle, great bomb" , "Small particle of matter" , "Moat (anag.)" , "Minute fragment" , "Elementary particle" .)