Quietly left for America, furthermore (4)
I believe the answer is:
plus
'furthermore' is the definition.
(I've seen this in another clue)
'quietly left for america' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'left' becomes 'l' (common abbreviation).
'for' is a charade indicator (letters next to each other).
'america' becomes 'US' (United States).
'p'+'l'+'us'='PLUS'
(Other definitions for plus that I've seen before include "In addition, also" , "advantageous" , "Added extra" , "Positive - addition" , "Furthermore" .)