Ingredient in soap (3)
I believe the answer is:
lye
'ingredient in soap' 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 "Strongly alkaline solution for washing or cleansing" , "Strongly alkaline wash" , "Alkali washing solution" , "Alkaline liquid" , "Cleansing alkaline fluid" .)