Quietly left us with a mathematical calculation (4)
I believe the answer is:
plus
'calculation' is the definition.
(I know that plus is a type of arithmetic operation)
'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 mathematical' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for plus that I've seen before include "mark for arithmetic" , "The opposite of minus" , "Furthermore" , "And also; a bonus" , "In addition, also" .)