A little matter that's positively elementary (6)
I believe the answer is:
proton
'a little' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'matter that's positively elementary' is the wordplay.
I cannot really see how this works, but
'matter' could be 'pr' (Pr is an example) and 'pr' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for proton that I've seen before include "Stable subatomic particle" , "One's charged" , "Particle in atoms" , "Nuclear particle with positive charge" , "Atomic constituent" .)