Crowd noise is crude we hear (4)
I believe the answer is:
roar
'crowd noise' is the definition.
(I've seen this in another clue)
'crude we hear' is the wordplay.
'crude' becomes 'raw' (synonyms).
'we hear' shows a homophone (sound like).
'raw' is a homophone of 'ROAR'.
'is' acts as a link.
(Other definitions for roar that I've seen before include "Lion's noise" , "The __ of the Greasepaint -- The Smell of the Crowd (1964 musical)" , "Yell" , "Bawl, bellow" , "Make a loud noise" .)