Quietly left us with a mathematical symbol (4)
I believe the answer is:
plus
'mathematical symbol' is the definition.
(I know that plus is a type of plus and minus signs)
'quietly left us' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'left' becomes 'l' (common abbreviation).
'p'+'l'+'us'='PLUS'
'with a' is the link.
(Other definitions for plus that I've seen before include "Positive - addition" , "Benefit" , "The opposite of minus" , "Distant" , "To the French it's more" .)