Vitamin B3 or nicotinic acid (6)
I believe the answer is:
niacin
I believe this is a double definition.
'vitamin b3' is the first definition.
(I've seen this in another clue)
'nicotinic acid' is the second definition.
(synonyms)
(Other definitions for niacin that I've seen before include "Another name for nicotinic acid" , "Type of vitamin" , "Vitamin, nicotinic acid" , "B vitamin which assists the nervous system" , "An essential B vitamin in Cain, strangely" .)