Ruins of French settlements vacated outside city of Paris (8)
I believe the answer is:
destroys
'ruins' is the definition.
(to destroy is to ruin or break)
'of french settlements vacated outside city of paris' is the wordplay.
'of french' becomes 'de' ('of' in French).
'vacated' suggests removing the centre (internal letters vacate the word).
'outside' means one lot of letters goes inside another.
'city of paris' becomes 'troy' (Paris was a hero from Troy in mythology).
'settlements' with its middle removed is 'ss'.
'ss' enclosing 'troy' is 'stroys'.
'de'+'stroys'='DESTROYS'
(Other definitions for destroys that I've seen before include "Demolishes, ruins" , "Spoils or ruins completely" , "Spoils or breaks completely" , "undoing" , "Ruins or bankrupts" .)