Article in Libération covering Northern French city (4)
I believe the answer is:
lens
'french city' is the definition.
(Lens is a French city)
'article in liberation covering northern' is the wordplay.
'article in liberation' becomes 'les' (I can't justify this - if you can you should give a lot more credence to this answer).
'covering' is an insertion indicator.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'les' enclosing 'n' is 'LENS'.
Can you help me to learn more?
(Other definitions for lens that I've seen before include "Focussing device" , "something to see right through" , "Refracting device" , "Piece of glass in optical instrument" , "Light-focusing device" .)