Third person tipped trendy vitamin (6)
I believe the answer is:
niacin
'vitamin' is the definition.
(I know that niacin is a type of B)
'third person tipped trendy' is the wordplay.
'third person' becomes 'cain' (biblical third person after Adam and Eve).
'tipped' says the letters should be written in reverse.
'trendy' becomes 'in' ('in' can mean fashionable).
'cain' reversed gives 'niac'.
'niac'+'in'='NIACIN'
(Other definitions for niacin that I've seen before include "Another name for nicotinic acid" , "An essential B vitamin in Cain, strangely" , "Vitamin, nicitinic acid" , "B vitamin which assists the nervous system" , "Vitamin, nicotinic acid" .)