Smallest unit of matter (4)
I believe the answer is:
atom
'smallest unit of matter' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for atom that I've seen before include "Particle of matter now divided" , "Smallest amount" , "Bit of make-up" , "It was once thought to be smallest particle of matter" , "An explosive particle as in bomb" .)