They're of little matter (5)
I believe the answer is:
atoms
'little matter' is the definition.
'atoms' can be an answer for 'matter' (thesaurus). I am unsure of the 'little' bit.
'they're of' is the wordplay.
I cannot quite understand how this works, but
'of' could be 'o' and 'o' is located in the answer.
The remaining letters 'atms' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for atoms that I've seen before include "Minute particles" , "Dots" , "Small particles" , "Little things" , "Tiny particles" .)