Sulphuric acid (7)
I believe the answer is:
vitriol
'sulphuric acid' is the definition.
(synonyms)
This is all the clue.
(Other definitions for vitriol that I've seen before include "Scathing criticism - sulphuric acid" , "Caustic words" , "Sulphuric acid or savage comment" , "Sulphuric acid and hate-filled criticism" , "Sulphuric acid; malicious criticism" .)