A cat, a tiny thing (4)
I believe the answer is:
atom
'a tiny thing' 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 "Smallest particle in an element able to take part in a chemical reaction" , "Particle of matter now divided" , "Small amount of matter" , "Smallest amount" , "Minute fragment" .)