A calculated balance (8)
I believe the answer is:
equation
'balance' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for equation that I've seen before include "Maths statement with =" , "Mathematical statment that two are equal" , "V=IR, say" , "feature of maths" , "Mathemical statement using = sign" .)