Low-pH substance (4)
I believe the answer is:
acid
'low-ph substance' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for acid that I've seen before include "Tart, sour" , "Drug LSD (colloq.)" , "It is neutralised by alkali" , "scathing" , "It is neutralized by alkali" .)