Nasty infestation tormenting scalps, primarily! (4)
I believe the answer is:
nits
'nasty infestation tormenting scalps primarily' is the definition.
I can't tell whether this definition defines the answer.
'nasty infestation tormenting scalps primarily' is the wordplay.
'primarily' suggests taking the first letters.
The first letters of 'nasty infestation tormenting scalps' is 'NITS'.
Can you help me to learn more?
(Other definitions for nits that I've seen before include "Tins of parasitic insect eggs" , "Eggs of lice" , "Fools" , "Head lice" , "Eggs; stupid people" .)