Female soldier joining love movement (6)
I believe the answer is:
adagio
'movement' is the definition.
(type of musical movement)
'female soldier joining love' is the wordplay.
'female' becomes 'Ada' (woman's name).
'soldier' becomes 'GI' (term for US soldier).
'joining' is a charade indicator (letters next to each other).
'love' becomes 'o' (love means zero in tennis).
'ada'+'gi'+'o'='ADAGIO'
(Other definitions for adagio that I've seen before include "Slowly and graceful, in music" , "In slow time (music)" , "Slowly, from a conductor" , "Musically, in slow time" , "develops slowly" .)