Wader gripped diver (7)
I believe the answer is:
bittern
'wader' is the definition.
(bittern is a kind of wader)
'gripped diver' is the wordplay.
'gripped' becomes 'bit' (biting is a kind of gripping).
'diver' becomes 'tern' (type of diving bird).
'bit'+'tern'='BITTERN'
(Other definitions for bittern that I've seen before include "Wading bird" , "Compact tawny-brown heron with nocturnal habits and a booming cry" , "Bird that booms" , "inhabitant of marsh" , "''He shall not hear the ... cry / In the wild sky where he is lain'' (Lament for Thomas MacDonagh)" .)