Roar like a bull (6)
I believe the answer is:
bellow
'roar like a bull' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for bellow that I've seen before include "Loud and deep roar" , "Deep roar as of bull" , "Loud deep sound as by bull" , "Roar, shout" , "Roar like a bull" .)