Critic and I regrettably offering caustic stuff (6,4)
I believe the answer is:
nitric acid
'caustic stuff' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'critic and i regrettably' is the wordplay.
'regrettably' indicates anagramming the letters.
'critic'+'and'+'i'='criticandi'
'criticandi' anagrammed gives 'NITRIC ACID'.
'offering' is the link.
Can you help me to learn more?
(Other definitions for nitric acid that I've seen before include "Corrosive liquid" , "dangerous stuff?" , "component of explosive" .)