Exaggerate the importance of the Louvre and rave about losing top rating (9)
I believe the answer is:
overvalue
'exaggerate' is the definition.
(thesaurus)
'about losing top rating' is the wordplay.
'about' becomes 'over' (both can mean around).
'losing top rating' becomes 'value' (I can't justify this - if you can you should believe this answer much more).
'over'+'value'='OVERVALUE'
'the importance of the louvre and rave' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for overvalue that I've seen before include "Rate worth too highly" , "Think too highly of" .)