Ace song employs two pianos put to practical use (7)
I believe the answer is:
applied
'put to practical use' is the definition.
(I've seen this before)
'ace song' is the wordplay.
'ace' becomes 'app' (I can't justify this - if you can you should believe this answer much more).
'song' becomes 'lied' (lied is a kind of song).
'app'+'lied'='APPLIED'
'employs two pianos' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for applied that I've seen before include "Assigned < practical" , "Responded to job ad" , "Was relevant" , "Practical rather than theoretical" , "sort of maths" .)