A little matter for one to get on with (3)
I believe the answer is:
ion
'a little matter' is the definition.
'ion' can be an answer for 'matter' (ion is a kind of matter). I'm not certain of the 'a little' bit.
'one to get on with' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'on' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'for' is the link.
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" .)