Soapmaking substance (3)
I believe the answer is:
lye
'soapmaking substance' 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 "Solution obtained by leaching" , "solution that comes out in the wash?" , "Alkaline liquid" , "Branch railway" , "Alkaline solution used for washing" .)