Extremely vocal about international group getting caustic criticism (7)
I believe the answer is:
vitriol
'caustic criticism' is the definition.
(I've seen this before)
'extremely vocal about international group' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'about' means one lot of letters goes inside another.
'international' becomes 'i' (abbreviation eg in IMF).
'group' becomes 'trio' (a group of three).
'vocal' with its middle removed is 'vl'.
'i'+'trio'='itrio'
'vl' placed around 'itrio' is 'VITRIOL'.
'getting' is the link.
(Other definitions for vitriol that I've seen before include "Caustic words" , "Abusive language - sulphuric acid" , "could be thrown in your face" , "Sulphuric acid or savage comment" , "Bitter words" .)