Soapmaking ingredient (3)
I believe the answer is:
lye
'soapmaking ingredient' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lye that I've seen before include "solution that comes out in the wash?" , "Alkaline solution used for washing" , "Strongly alkaline wash" , "Alkali washing solution" , "Solution obtained by leaching" .)