Set my terms with variable balance (8)
I believe the answer is:
symmetry
'balance' is the definition.
(I know that balance is a class)
'set my terms with variable' is the wordplay.
'set' indicates anagramming the letters (instruction to set the letters).
'with' means one lot of letters go next to another.
'variable' becomes 'y' (algebra).
'my'+'terms'='myterms'
'myterms' anagrammed gives 'symmetr'.
'symmetr'+'y'='SYMMETRY'
(Other definitions for symmetry that I've seen before include "Exact correspondence between parts" , "Pleasing proportion between parts of a whole" , "Mirrored disposition about axis" , "Pleasing proportion of the parts of a thing" , "Proportion; beauty of form" .)