Quantum of light (6)
I believe the answer is:
photon
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">quantum of light</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for photon that I've seen before include "Tiny bit of light" , "A little light" , "Quantum particle of light" , "Quantum of electromagnetic radiation" .)