A rye 'Webster's' set out as a source of vitamin B (7,5)
I believe the answer is:
brewers yeast
'a source of vitamin b' is the definition.
(brewer's yeast contains vitamin B)
'a rye webster's set out' is the wordplay.
'set out' indicates an anagram.
'a'+'rye'+'websters'='aryewebsters'
'aryewebsters' is an anagram of 'BREWERS YEAST'.
'as' acts as a link.
(Other definitions for brewers yeast that I've seen before include "Ingredient used in beer production" , "Fermenting agent in liquor" .)