Old American round piano in musical work (4)
I believe the answer is:
opus
'musical work' is the definition.
(synonyms)
'old american round piano' is the wordplay.
'old' becomes 'o' (abbreviation).
'american' becomes 'us' (abbreviation for United States).
'round' indicates putting letters inside.
'piano' becomes 'p' (musical abbreviation).
'o'+'us'='ous'
'ous' placed around 'p' is 'OPUS'.
'in' acts as a link.
(Other definitions for opus that I've seen before include "Piece of music (with number)" , "Work (of a composer?)" , "(Numbered) musical composition" , "Work (mus.)" , "Creative work, eg in writing, art or music" .)