Extremely corrosive substance brought up during a meeting (9)
I believe the answer is:
radically
'extremely' is the definition.
(thesaurus)
'corrosive substance brought up during a meeting' is the wordplay.
'corrosive substance' becomes 'acid' (I've seen this in another clue).
'brought up' says the letters should be written in reverse (in a down clue, letters go up).
'during' is an insertion indicator.
'a meeting' becomes 'rally' (I have seen 'Mass meeting ' mean 'rally' so perhaps 'meeting' could also mean 'rally').
'acid' reversed gives 'dica'.
'dica' placed into 'rally' is 'RADICALLY'.
(Other definitions for radically that I've seen before include "Fundamentally" , "seriously" , "in a serious way" , "Essentially" .)