Louse's egg (3)
I believe the answer is:
nit
'louse's egg' is the definition.
(I've seen this before)
This is all the clue.
(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" .)