The United Kingdom united over two French articles - on which strings are attached (7)
I believe the answer is:
ukulele
'on which strings are attached' is the definition.
(stringed instrument)
'the united kingdom united over two french articles' is the wordplay.
'the united kingdom' becomes 'uk' (synonyms).
'united' becomes 'u'.
'over' means one lot of letters go next to another.
'two' indicates the doubling of some letters.
'french articles' becomes 'le' (I can't justify this - if you can you should believe this answer much more).
'le' duplicated is 'lele'.
'uk'+'u'+'lele'='UKULELE'
(Other definitions for ukulele that I've seen before include "play this?" , "Small, four-stringed guitar" , "One needs pluck in this" , "Stringed instrument" , "Small Hawaiian guitar" .)