Seabird uncommon in Atlantic state (5)
I believe the answer is:
noddy
'seabird' is the definition.
(noddy is a type of bird)
'uncommon in atlantic state' is the wordplay.
'uncommon' becomes 'odd' (odd can mean unexpected or unusual).
'in' means one lot of letters goes inside another.
'atlantic state' becomes 'NY' (New York is on the US' Atlantic coast).
'odd' placed within 'ny' is 'NODDY'.
(Other definitions for noddy that I've seen before include "Tropical tern" , "Enid Blyton creation" , "Old fool" , "Simpleton; oil rig" , "children's character" .)