Wading bird nipped a gull's relative (7)
I believe the answer is:
bittern
'wading bird' is the definition.
(bittern is a kind of wading bird)
'nipped a gull's relative' is the wordplay.
'nipped' becomes 'bit' (I can't justify this - if you can you should believe this answer much more).
'a gull's relative' becomes 'tern' (I've seen this in another clue).
'bit'+'tern'='BITTERN'
Can you help me to learn more?
(Other definitions for bittern that I've seen before include "'He shall not hear the . . . cry' (Ledwidge)" , "Source of boom" , "Marshland bird" , "Marsh bird of the heron family" , "Aquatic bird" .)