Vitamin B1 (7)
I believe the answer is:
thiamin
'vitamin b1' is the definition.
(I know that vitamin b1 can be written as Thiamin)
This is the entire clue.
(Other definitions for thiamin that I've seen before include "Vitamin B 1" , "Vitamin in cereals, beans, etc" , "Essential nutrition" , "vitamin required" , "food ingredient" .)