Bellow, roar (7)
I believe the answer is:
thunder
I believe this is a double definition.
'bellow' is the first definition.
(I've seen this in another clue)
'roar' is the second definition.
(thundering is a kind of roaring)
(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" .)