Riviera gendarme has this secret plan (6,6)
I believe the answer is:
hidden agenda
'plan' is the definition.
(I have seen 'covert plan' mean 'hidden agenda' so perhaps 'plan' could also mean 'hidden agenda')
'riviera gendarme has this secret' is the wordplay.
'riviera gendarme' becomes 'agenda' (I can't explain this - if you can you should give a lot more credence to this answer).
'has' is a charade indicator (letters next to each other).
'this secret' becomes 'hidden' (I've seen this before).
'agenda' put after 'hidden' is 'HIDDEN AGENDA'.
Can you help me to learn more?
(Other definitions for hidden agenda that I've seen before include "followed in secret meeting?" , "Ulterior motive" , "Sub-text, unstated objective" , "Concealed intentions" , "covert plan" .)