Retreat in outskirts of Paris comes before (8)
I believe the answer is:
precedes
'comes before' is the definition.
('precede' can be a synonym of 'come before')
'retreat in outskirts of paris' is the wordplay.
'retreat' becomes 'recede' (receding is a kind of retreating).
'in' means one lot of letters goes inside another.
'outskirts of paris' becomes 'ps' (first and last letters of 'Paris').
'recede' going inside 'ps' is 'PRECEDES'.
(Other definitions for precedes that I've seen before include "Heralds" , "Comes before" , "Goes before in time or order" , "Goes in front (of)" , "has priority" .)