Clue: Nasty infestation tormenting scalps, primarily! (4)
Answer: NITS
Explanation: Nasty infestation tormenting scalps, primarily! (4)
First letters ('primarily') of '**N**asty **I**nfestation **T**ormenting **S**calps' &lit (but to be pedantic, it is not the nits which torment, but the lice which hatch from them – OK, Chambers does give an alternate meaning of nit as a young louse).