British and French are adopting long trousers (8)
I believe the answer is:
britches
'trousers' is the definition.
(I know this)
'british and french are adopting long' is the wordplay.
'british' becomes 'br' (abbreviation for British).
'and' means one lot of letters go next to another.
'french are' becomes 'es' ('are' in French).
'adopting' is an insertion indicator (inserted letters are taken on or adopted).
'long' becomes 'itch' (to long or itch for something is to desire it).
'br'+'es'='bres'
'bres' enclosing 'itch' is 'BRITCHES'.
(Other definitions for britches that I've seen before include "American garment" , "Trousers" , "Garb for Yanks" .)