Attorney and soldier follow key at old tempo (6)
I believe the answer is:
adagio
'old tempo' is the definition.
'adagio' can be an answer for 'tempo' (I have seen 'Slow tempo ' mean 'adagio' so perhaps 'tempo' could also mean 'adagio'). I am not sure about the 'old' bit.
'attorney and soldier follow key' is the wordplay.
'attorney' becomes 'DA' (District Attorney).
'and' is a charade indicator (letters next to each other).
'soldier' becomes 'gio' (I can't explain this - if you can you should believe this answer much more).
'follow' is a charade indicator (letters next to each other).
'key' becomes 'A' (musical key).
'da' put next to 'gio' is 'dagio'.
'dagio' after 'a' is 'ADAGIO'.
'at' is the link.
Can you help me to learn more?
(Other definitions for adagio that I've seen before include "Slowly and graceful, in music" , "Slow movement in musical work" , "Musically, in slow time" , "(Piece to be played) slowly" , "Slow movement (music)" .)