Crossword Genius

Unexpected retreat in the south of France? (6)

Ross

I believe the answer is:

sudden

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'unexpected' is the definition.
(similar in meaning)

'retreat in the south of france?' is the wordplay.
I cannot quite understand how this works, but
'retreat' could be 'den' (both can mean a lair or hideout) and 'den' is present in the answer.
'south' could be 's' (synonyms) and 's' is located in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for sudden that I've seen before include "Quickly, without notice or warning" , "Quick and unexpected" , "Occurring unexpectedly quickly" , "'Unexpectedly, without warning (6)'" , "Happening abruptly" .)

I've seen this clue in The Guardian.
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