Sugarloaf Mountain city (3)
I believe the answer is:
rio
'sugarloaf mountain 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 "Erstwhile home of Biggs" , "(Golfer) in the sand" , "A Mexican river - like Grande" , "The ... Grande borders Texas and Mexico" , "Brazilian city and port" .)