Potty about American work of music? (4)
I believe the answer is:
opus
'work of music?' is the definition.
(opus can mean a work of classical music)
'potty about american' is the wordplay.
'potty' becomes 'po' (I've seen this in other clues).
'about' says the letters should be written in reverse.
'american' becomes 'us' (abbreviation for United States).
'po' backwards is 'op'.
'op'+'us'='OPUS'
(Other definitions for opus that I've seen before include "Musical composition in the soup" , "Piece of music (with number)" , "A Latin work" , "Work of 25 perhaps" , "Work (of a composer?)" .)