B complex vitamin (6)
I believe the answer is:
niacin
'b complex vitamin' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for niacin that I've seen before include "B vitamin which assists the nervous system" , "Vitamin, nicitinic acid" , "Another name for nicotinic acid" , "Vitamin, nicotinic acid" , "Type of vitamin" .)