Vitamin is weak, concealing different aim (7)
I believe the answer is:
thiamin
'vitamin' is the definition.
(I know that thiamin is a type of B)
'weak concealing different aim' is the wordplay.
'weak' becomes 'thin' (similar in meaning).
'concealing' means one lot of letters goes inside another.
'different' indicates an anagram (letters placed in a different order).
'aim' anagrammed gives 'ami'.
'thin' going around 'ami' is 'THIAMIN'.
'is' acts as a link.
(Other definitions for thiamin that I've seen before include "food ingredient" , "Essential nutrition" , "Vitamin B 1" , "vitamin required" , "Vitamin in cereals, beans, etc" .)