A little matter for one to get on with (3)
I believe the answer is:
ion
'a little' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'matter for one to get on with' is the wordplay.
'matter' becomes 'N' (N is an example).
'for' says to put letters next to each other (I've seen this in other clues).
'one to get on with' becomes 'io' (I can't justify this - if you can you should give a lot more credence to this answer).
'n' after 'io' is 'ION'.
Can you help me to learn more?
(Other definitions for ion that I've seen before include "Charged atom/molecule" , "Electrically charged particle" , "Particle (phys.)" , "Subatomic particle" , "Eletrically charged particle" .)