Baking powder ingredient (5,2,6)
I believe the answer is:
cream of tartar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">baking powder ingredient</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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?" .)