Bellow, roar (7)
I believe the answer is:
thunder
I believe this clue is a double definition.
'bellow' is the first definition.
(I've seen this in another clue)
'roar' is the second definition.
(synonyms)
(Other definitions for thunder that I've seen before include "Red hunt for a loud noise" , "Heavenly rumbling" , "Loud rumbling noise" , "Noise heard after lightning" , "Noise named after the god Thor" .)