Principality on the French Riviera (6)
I believe the answer is:
monaco
'principality on the french riviera' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for monaco that I've seen before include "State of the Grimaldis" , "Principality of the Rainiers" , "Second smallest sovereign state" , "Home of the Grimaldis" , "Southwest European principality" .)