A cat's little bits? (5)
I believe the answer is:
atoms
'little bits?' is the definition.
(I've seen this in another clue)
'a cat's' is the wordplay.
'cat's' becomes 'tom's' (a tom is a male cat).
'a'+'toms'='ATOMS'
(Other definitions for atoms that I've seen before include "Jots" , "Dots" , "small fragments" , "Minute particles" , "Tiny particles" .)