Scrap with a cat (4)
I believe the answer is:
atom
'scrap' is the definition.
(both can mean a tiny amount)
'with a cat' is the wordplay.
'with' is a charade indicator (letters next to each other).
'cat' becomes 'tom' (a tom is a male cat).
'a' next to 'tom' is 'ATOM'.
(Other definitions for atom that I've seen before include "The tiniest particle" , "Bit of make-up" , "Scrap" , "Something very small perhaps" , "A wee bit" .)