At sea, saw the rare seabird (10)
I believe the answer is:
shearwater
'seabird' is the definition.
(shearwater is a kind of seabird)
'at sea saw the rare' is the wordplay.
'at sea' indicates anagramming the letters.
'saw'+'the'+'rare'='sawtherare'
'sawtherare' with letters rearranged gives 'SHEARWATER'.
(Other definitions for shearwater that I've seen before include "Manx --, seabird" , "Bird" .)