Particle of matter (4)
I believe the answer is:
atom
'particle of matter' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for atom that I've seen before include "A tiny powerful particle" , "Smallest unit of matter" , "A tiny particle of matter with a big bang" , "Something very small perhaps" , "Moat (anag.)" .)