Substance with a low pH (4)
I believe the answer is:
acid
'substance with a low ph' 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 "It is neutralised by alkali" , "Drug, tart" , "Cutting" , "It is neutralized by alkali" , "Corrosive substance" .)