Extreme talk supported by a majority (9)
I believe the answer is:
uttermost
'extreme' is the definition.
(thesaurus)
'talk supported by a majority' is the wordplay.
'talk' becomes 'utter' ('utter' can be a synonym of 'talk'**).
'supported by' means one lot of letters go next to another (letters go below - in down clue).
'a majority' becomes 'most' (I've seen this before).
'utter'+'most'='UTTERMOST'
(Other definitions for uttermost that I've seen before include "The furthest point or degree" , "best" , "Extreme" , "Last" , "Chief" .)