Baking powder ingredient (5,2,6)
I believe the answer is:
cream of tartar
'baking powder ingredient' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cream of tartar that I've seen before include "White powder used in baking" , "Cooking ingredient" , "Salt used in baking powder" , "An ingredient of baking powder" , "cake baked with this?" .)