Low-pH chemical (4)
I believe the answer is:
acid
'low-ph chemical' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for acid that I've seen before include "something sharp" , "So-called drug" , "- - -drop, sweet" , "Sharp - tart - kind of house party" , "Alkali's opposite" .)