Sounds a bit obscure, but it's a sea bird (4)
I believe the answer is:
skua
'it's a sea bird' is the definition.
(skua is a kind of sea bird)
'sounds a bit obscure' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
The remaining letters 'sku' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'but' is the link.
Can you help me to learn more?
(Other definitions for skua that I've seen before include "Seabird; sounds like pin" , "Arctic seabird" , "Large brown seabird" , "One preying on gulls" , "Large sea bird" .)