Old piano introducing American piece of music (4)
I believe the answer is:
opus
'piece of music' is the definition.
('opus' can be a synonym of 'piece of music')
'old piano introducing american' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'piano' becomes 'p'.
'introducing' says to put letters next to each other.
'american' becomes 'us'.
'o'+'p'+'us'='OPUS'
(Other definitions for opus that I've seen before include "Artistic work" , "Soup (anag.)" , "Composer's catalogue number" , "A Latin work" , "Work (mus.)" .)