One has left scent with a melodic expression (3-2)
I believe the answer is:
tra-la
'melodic expression' is the definition.
(syllables representing song)
'one has left scent with a' is the wordplay.
'one' becomes 'I' (Roman numeral).
'has left' suggests deleting specific letters.
'scent' becomes 'trail' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'trail' with 'i' taken out is 'tral'.
'tral'+'a'='TRA-LA'
(Other definitions for tra-la that I've seen before include "The flowers that bloom in the spring -- -- (Mikado)" , "Sound made when humming a song" , "Joyous refrain in song" , "Ironic; expression of joy or gaiety" , "Expression of joy or gaiety, musically" .)