Another name for nicotinic acid, whose deficiency in the diet causes pellagra (6)
I believe the answer is:
niacin
(Other definitions for niacin that I've seen before include "An essential B vitamin in Cain, strangely" , "Vitamin, nicitinic acid" , "Vitamin, nicotinic acid" , "B vitamin which assists the nervous system" , "Type of vitamin" .)