World's rarest goose (4)
I believe the answer is:
nene
'world's rarest goose' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for nene that I've seen before include "Flower" , "Northants river" , "Northampton's river" , "River navigable to Northampton" , "River of Peterborough; goose" .)