Drain-cleaner ingredient (3)
I believe the answer is:
lye
'drain-cleaner 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 "Strongly alkaline wash" , "Alkali washing solution" , "Solution obtained by leaching" , "Strong alkaline solution" , "Alkaline solution used for washing" .)