Summer Olympics host between London and Tokyo (3)
I believe the answer is:
rio
'summer olympics host between london and tokyo' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rio that I've seen before include "Manchester United player in South American city, in short?" , "The ... Grande borders Texas and Mexico" , "... Grande, ... de Janeiro" , "A Mexican river, may be grande" , "place of carnival" .)