Egg of a louse (3)
I believe the answer is:
nit
'egg of a louse' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nit that I've seen before include "Egg of a head louse" , "Parasite's egg made of tin" , "Louse's egg; idiot" , "Louse; fool" , "Louse-egg" .)