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