Elemental building block (4)
I believe the answer is:
atom
'elemental building block' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for atom that I've seen before include "Something very small perhaps" , "A really tiny piece of something" , "Very small bit" , "The tiniest particle" , "A mighty particle, makes huge explosion" .)