One in the balance on horse almost balances (10)

'balances' is the definition.
('equilibrium' can be a synonym of 'balance')

'one in the balance on horse almost' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in the' means one lot of letters goes inside another.
'balance' becomes 'libra' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'horse almost' becomes 'equi' (I can't justify this - if you can you should give a lot more credence to this answer).
'i' put inside 'libra' is 'libria'.
'libria' put after 'equi' is 'EQUILIBRIA'.