A cat is a tiny little thing (4)
I believe the answer is:
atom
'a tiny little thing' is the definition.
(tiny particle)
'a cat' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'a'+'tom'='ATOM'
'is' acts as a link.
(Other definitions for atom that I've seen before include "Basic unit of a chemical element" , "It was once thought to be smallest particle of matter" , "Very small object or amount" , "Particle of matter now divided" , "A tiny powerful particle" .)