Industrial waste slick on west-facing borders of France (8)
I believe the answer is:
effluent
'industrial waste' is the definition.
(effluent is liquid waste)
'slick on west-facing borders of france' is the wordplay.
'slick' becomes 'fluent' (fluent can mean slick or smooth).
'on' says to put letters next to each other.
'west-facing' says the letters should be written backwards (letters written right to left or east to west).
'borders of' means to remove the middle letters.
'france' with its middle taken out is 'fe'.
'fe' backwards is 'ef'.
'fluent' after 'ef' is 'EFFLUENT'.
(Other definitions for effluent that I've seen before include "Waste water" , "Outflow of liquid waste matter" , "contents of 25 [SEWER]" , "It flows and contaminates" , "sewage" .)