Booming bird (7)
I believe the answer is:
bittern
'booming bird' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for bittern that I've seen before include "Aquatic bird" , "Compact tawny-brown heron with nocturnal habits and a booming cry" , "Source of boom" , "found hidden in reed-beds" , "Wading bird" .)