Express disapproval with mass roar (4)
I believe the answer is:
boom
'roar' is the definition.
(synonyms)
'express disapproval with mass' is the wordplay.
'express disapproval' becomes 'boo' (as in a crowd booing).
'with' is a charade indicator (letters next to each other).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'boo'+'m'='BOOM'
(Other definitions for boom that I've seen before include "Resound - spar - economic growth" , "Harbour barrier" , "Prosper vigorously - waterway barrier" , "When business is great, opposite to bust" , "Pole for overhead microphone - deep sound" .)