Itsy-bitsy bit (4)
I believe the answer is:
atom
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">itsy-bitsy bit</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for atom that I've seen before include "Very small object or amount" , "A wee bit" , "A tiny particle of matter with a big bang" , "Tiny bit" , "Particle of matter now divided" .)