Roar, yell (6)
I believe the answer is:
bellow
This clue is a double definition.
'roar' is the first definition.
(I know that roar can be written as bellow)
'yell' is the second definition.
(bellowing is a kind of yelling)
(Other definitions for bellow that I've seen before include "Loud deep sound as by bull" , "Roar loudly" , "Roar like a bull" , "Loud cry of a bull" , "Loud and deep roar" .)