Vitamin is first of many found in tahini spread (7)
I believe the answer is:
thiamin
'vitamin' is the definition.
(I know that thiamin is a type of B)
'first of many found in tahini spread' is the wordplay.
'first of many' becomes 'm' (1st letter of 'many').
'found in' means one lot of letters goes inside another.
'spread' is an anagram indicator.
'tahini' is an anagram of 'thiain'.
'm' put into 'thiain' is 'THIAMIN'.
'is' is the link.
(Other definitions for thiamin that I've seen before include "Vitamin in cereals, beans, etc" , "food ingredient" , "Essential nutrition" , "Vitamin B 1" , "vitamin required" .)