Mostly overcast, grand French city (8)
I believe the answer is:
grenoble
'french city' is the definition.
(city in southeastern France)
'mostly overcast grand' is the wordplay.
'mostly overcast' becomes 'gre' (I can't justify this - if you can you should give a lot more credence to this answer).
'grand' becomes 'noble' (similar in meaning).
'gre'+'noble'='GRENOBLE'
Can you help me to learn more?
(Other definitions for grenoble that I've seen before include "European city" , "City of southeast France" , "French city and skiing centre" , "French Alpine city" , "City of south-eastern France" .)