Huge demo turns nasty around the middle of Whitehall (10)
I believe the answer is:
tremendous
'huge' is the definition.
(synonyms)
'demo turns nasty around the middle of whitehall' is the wordplay.
'nasty' indicates anagramming the letters.
'around' is an insertion indicator.
'the middle of' indicates the central letters.
The central letter of 'whitehall' is 'e'.
'demo'+'turns'='demoturns'
'demoturns' is an anagram of 'tremndous'.
'tremndous' placed around 'e' is 'TREMENDOUS'.
(Other definitions for tremendous that I've seen before include "Immense" , "Wonderful" , "large" , "Splendid!" , "Fantastic" .)