The balance mounts (6)
I believe the answer is:
scales
I believe this clue is a double definition.
'the balance' is the first definition.
(I've seen this in another clue)
'mounts' is the second definition.
(scaling is a kind of mounting)
(Other definitions for scales that I've seen before include "Flakes of fish skin" , "Set of balances" , "They're found on fish" , "Fish flakes or measuring devices" , "Richter etc?" .)