Soapmaker's ingredient (3)
I believe the answer is:
lye
'soapmaker's ingredient' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lye that I've seen before include "Branch railway" , "Alkali washing solution" , "Cleansing alkaline fluid" , "Strongly alkaline solution for washing or cleansing" , "solution that comes out in the wash?" .)