Second pale bird (4)
I believe the answer is:
swan
'bird' is the definition.
(I know that swan is a type of aquatic bird)
'second pale' is the wordplay.
'second' becomes 's' (common abbreviation).
'pale' becomes 'wan' (wan can mean pale or grey).
's'+'wan'='SWAN'
(Other definitions for swan that I've seen before include "... Australian river" , "Sweet name 8 [OF SHAKESPEARE]" , "Whooper, perhaps" , "Large aquatic bird" , "The Ugly Duckling's outcome" .)