Morsel for a cat (4)
I believe the answer is:
atom
'morsel' is the definition.
(thesaurus)
'a cat' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'a'+'tom'='ATOM'
'for' acts as a link.
(Other definitions for atom that I've seen before include "tiny thing" , "A tiny particle of matter with a big bang" , "type of bomb" , "Not much substance" , "Smallest component of an element" .)