Express disapproval with masculine roar (4)
I believe the answer is:
boom
'roar' is the definition.
(I know that roar can be written as boom)
'express disapproval with masculine' is the wordplay.
'express disapproval' becomes 'boo' (as in a crowd booing).
'with' means one lot of letters go next to another.
'masculine' becomes 'm'.
'boo'+'m'='BOOM'
(Other definitions for boom that I've seen before include "Resound - spar - economic growth" , "Prosperous time -- rumbling sound" , "Dull explosion; prosperous period" , "Resonant sound, like artillery" , "Prosperity - hollow roaring noise" .)