A cat can mean very little (4)
I believe the answer is:
atom
'mean very little' is the definition.
'atom' can be an answer for 'little' (thesaurus). I'm not certain of the 'mean very' bit.
'a cat' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'a'+'tom'='ATOM'
'can' is the link.
Can you help me to learn more?
(Other definitions for atom that I've seen before include "Part of molecule" , "A tiny particle but a big bomb" , "Minute particle" , "A really tiny piece of something" , "The tiniest particle" .)