Balance arm is caught in edges of one (9)

'balance' is the definition.
(I know that balance can be written as equipoise)

'arm is caught in edges of one' is the wordplay.
'arm' becomes 'equip' (arm can mean to equip or supply).
'caught in' means one lot of letters goes inside another.
'edges of' means to remove the middle letters.
'one' with its centre taken out is 'oe'.
'is' going inside 'oe' is 'oise'.
'equip'+'oise'='EQUIPOISE'