Immature parasites (4)
I believe the answer is:
nits
'immature parasites' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for nits that I've seen before include "Tins of parasitic insect eggs" , "Head louse eggs" , "Head lice" , "Fools" , "Eggs of lice" .)