Great unrest with demo out of control (10)
I believe the answer is:
tremendous
'great' is the definition.
(thesaurus)
'unrest with demo out of control' is the wordplay.
'with' says to put letters next to each other.
'out of control' indicates an anagram.
'unrest'+'demo'='unrestdemo'
'unrestdemo' with letters rearranged gives 'TREMENDOUS'.
(Other definitions for tremendous that I've seen before include "large" , "Fantastic" , "Immense" , "Excellent; very great" , "Splendid!" .)