Louse's egg (3)
I believe the answer is:
nit
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">louse's egg</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for nit that I've seen before include "Cause of much head-scratching" , "Louse's egg; idiot" , "Young louse" , "Parasite's egg made of tin" , "Idiot, ass" .)