Joins up with the French, led by a top officer (7)
I believe the answer is:
couples
'joins' is the definition.
(coupling is a kind of joining)
'up with the french led by a top officer' is the wordplay.
'with' says to put letters next to each other.
'the french' becomes 'les' (plural form of 'the' in French).
'led by' means one lot of letters go next to another.
'a top officer' becomes 'CO' (commanding officer).
'up'+'les' is 'uples'.
'uples' after 'co' is 'COUPLES'.
(Other definitions for couples that I've seen before include "People with their partners" , "Pairs" .)