Soap-making need (3)
I believe the answer is:
lye
'soap-making need' 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 "Strongly alkaline solution for washing or cleansing" , "Strongly alkaline wash" , "caustic liquid" , "Alkaline solution used for washing" , "Branch railway" .)