Elemental unit (4)
I believe the answer is:
atom
'elemental unit' 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 "A tiny particle but a big bomb" , "Smallest component of an element" , "Tiny particle, great bomb" , "Something very small perhaps" , "Not much substance" .)