Crossword Genius

Sour substance (4)

Ross

I believe the answer is:

acid

'sour substance' 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 "Substance with pH of less than 7" , "Caustic and biting" , "Acerbic" , "- - -drop, sweet" , "Substance which turns litmus red" .)

I've seen this clue in the Penny Press and the Sydney Morning Herald.
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