Balance, due proportion (8)
I believe the answer is:
symmetry
I believe this clue is a double definition.
'balance' is the first definition.
(synonyms)
'due proportion' is the second definition.
(I've seen this before)
(Other definitions for symmetry that I've seen before include "Due proportion" , "Balance in appearance" , "Pleasing proportion of the parts of a thing" , "Arrangement of parts in pleasing proportions" , "shapely attribute" .)