Casual discussion about local area network underpins denial from France (10)
I believe the answer is:
nonchalant
'casual' is the definition.
(I know that casual can be written as nonchalant)
'discussion about local area network underpins denial from france' is the wordplay.
'discussion' becomes 'chat'.
'about' means one lot of letters goes inside another.
'local area network' becomes 'LAN' (computing abbreviation).
'underpins' says to put letters next to each other (in a down clue, some letters go underneath others).
'denial from france' becomes 'non' ('no' in French).
'chat' placed around 'lan' is 'chalant'.
'chalant' put after 'non' is 'NONCHALANT'.
(Other definitions for nonchalant that I've seen before include "Cool" , "Casually unconcerned" , "Insouciant" , "Casual" , "Calmly unconcerned" .)