Nicotinic acid (6)
I believe the answer is:
niacin
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">nicotinic acid</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for niacin that I've seen before include "An essential B vitamin in Cain, strangely" , "Type of vitamin" , "B vitamin which assists the nervous system" , "Vitamin, nicotinic acid" , "Another name for nicotinic acid" .)