Small building block (4)
I believe the answer is:
atom
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">small building block</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for atom that I've seen before include "Tiny bit" , "Something very small perhaps" , "Nucleus plus electrons" , "A really tiny piece of something" , "Minute particle" .)