Guanabara Bay city (3)
I believe the answer is:
rio
'guanabara bay city' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rio that I've seen before include "Flying Down to -, Astaire, Rogers film" , "old capital" , "Spanish word for river" , "Brazilian city and port" , "Manchester United player in South American city, in short?" .)