A little matter at the rationing centre (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.
'the rationing centre' is the wordplay.
'centre' says to take the centre.
The central letters of 'rationing' are 'ION'.
'at' 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.)" , "Electrically charged atom or group of atoms" , "Eletrically charged particle" .)