A cat -- ounce? (4)
I believe the answer is:
atom
'ounce?' is the definition.
(thesaurus)
'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 tiny powerful particle" , "Moat (anag.)" , "A small but mighty particle" , "Very small bit" , "Nucleus surrounded" .)