Tiny part of a cat (4)
'tiny part' is the definition.
'atom' can be an answer for 'part' (I have seen 'Part of molecule ' mean 'atom' so perhaps 'part' could also mean 'atom'). I am not certain of the 'tiny' bit.
'a cat' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'of' is the link.
Can you help me to learn more?
(Other definitions for atom that I've seen before include "Small amount of matter", "Small particle of matter", "Nucleus surrounded", "type of bomb", "Smallest particle in an element able to take part in a chemical reaction".)