Young louse (3)
I believe the answer is:
nit
'young louse' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for nit that I've seen before include "Egg; fool" , "Foolish person" , "Egg of parasitic insect" , "Cause of much head-scratching" , "Stupid person; egg" .)