Tiny building block (4)
I believe the answer is:
atom
'tiny building block' 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 "Basic unit of a chemical element" , "tiny thing" , "An explosive particle as in bomb" , "It was once thought to be smallest particle of matter" , "Particle of matter now divided" .)