Drain opener (3)
I believe the answer is:
lye
'drain opener' 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 "Alkali washing solution" , "Strongly alkaline solution for washing or cleansing" , "Solution obtained by leaching" , "Alkaline solution used for washing" , "caustic liquid" .)