Briefly spotted extinct bird in part of Polynesia (5)
I believe the answer is:
samoa
'part of polynesia' is the definition.
('Samoa' is part of 'polynesia')
'briefly spotted extinct bird' is the wordplay.
'briefly' means to remove the last letter.
'spotted' becomes 'saw' (spot can mean to see).
'extinct bird' becomes 'moa' (extinct flightless bird of New Zealand).
'saw' with its last letter taken off is 'sa'.
'sa'+'moa'='SAMOA'
'in' acts as a link.
(Other definitions for samoa that I've seen before include "island territory" , "Group of volcanic islands in the S Pacific" , "South-west Pacific nation" , "South Pacific island nation" , "Southwest Pacific island group" .)