Eg, proton (8)
I believe the answer is:
particle
'eg proton' is the definition.
(proton is a type of subatomic particle)
This is the entire clue.
(Other definitions for particle that I've seen before include "Grain, tiny piece" , "Speck of matter" , "Eg, a proton" , "Prelatic (anag.)" , "Tiny piece of anything" .)