Thunder (4)
I believe the answer is:
roar
'thunder' is the definition.
(I know that thunder can be written as roar)
This is the entire clue.
(Other definitions for roar that I've seen before include "Zoom - prolonged cry" , "Jaguar, for example, emits this" , "Lion's call" , "Deep-throated cry" , "Lion's sound" .)