Slowly American soldier supports girl with love (6)
I believe the answer is:
adagio
'slowly' is the definition.
(I've seen this before)
'american soldier supports girl with love' is the wordplay.
'american soldier' becomes 'gi'.
'supports' says to put letters next to each other (bottom letters provide a support for the top ones).
'girl' becomes 'ada' ().
'with' says to put letters next to each other.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'gi' put after 'ada' is 'adagi'.
'adagi'+'o'='ADAGIO'
(Other definitions for adagio that I've seen before include "Slow musical piece or movement" , "Slowly and gracefully, musically" , "'Slowly and gracefully, in music (6)'" , "' - - for Strings' (Barber)" , "Slow musical passage" .)