Small heron (7)
I believe the answer is:
bittern
(Other definitions for bittern that I've seen before include "Compact tawny-brown heron with nocturnal habits and a booming cry" , "Source of boom" , "'He shall not hear the . . . cry' (Ledwidge)" , "''He shall not hear the ... cry / In the wild sky where he is lain'' (Lament for Thomas MacDonagh)" , "flier" .)