A little matter of national importance! (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.
'of national importance' is the wordplay.
'of' indicates a hidden word.
'ION' is hidden in the letters of 'national importance'.
Can you help me to learn more?
(Other definitions for ion that I've seen before include "Subatomic particle" , "Electrically charged atom or group of atoms" , "Electrically-charged particle" , "Electrically charged particle" , "Eletrically charged particle" .)