French city is mainly dull and worthy (8)

I believe the answer is:

grenoble

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'french city' is the definition.
(city in southeastern France)

'mainly dull and worthy' is the wordplay.
'mainly dull' becomes 'gre' (I am not sure about this - if you are sure you should believe this answer much more).
'and' means one lot of letters go next to another.
'worthy' becomes 'noble' (synonyms).
'gre'+'noble'='GRENOBLE'

'is' acts as a link.

Can you help me to learn more?

(Other definitions for grenoble that I've seen before include "European city" , "City of SE France" , "City of south-eastern France" , "City of southeast France" , "French Alpine city" .)

I've seen this clue in The Guardian.

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