Drain cleaner chemical (3)
I believe the answer is:
lye
'drain cleaner chemical' 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 "caustic liquid" , "Solution obtained by leaching" , "Alkali washing solution" , "Alkaline solution used for washing" , "Branch railway" .)