What's the matter? (4)
I believe the answer is:
atom
'what's the matter?' is the definition.
(atom is a kind of matter)
This is the entire clue.
(Other definitions for atom that I've seen before include "It was once thought to be smallest particle of matter" , "Piece of matter in moat" , "Nucleus surrounded" , "A tiny powerful particle" , "Particle of matter now divided" .)