Only half the characters, little matter (4)
I believe the answer is:
atom
I believe this clue is a double definition.
'only half the characters' is the first definition.
(A to M, the first 13 letters of the alphabet)
'little matter' is the second definition.
(tiny particle of matter)
(Other definitions for atom that I've seen before include "Very small object or amount" , "Tiny particle, great bomb" , "Smallest component of an element" , "A small but mighty particle" , "Minute particle" .)