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'm 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 "Basic unit of a chemical element", "Minute particle", "It was once thought to be smallest particle of matter", "type of bomb", "Very small object or amount".)