Old head of Orica is brilliant Australian (3)

'brilliant australian' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'old head of orica' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'head of' says to take the initial letters.
The first letter of 'orica' is 'o'.
'ex'+'o'='EXO'

'is' acts as a link.

Can you help me to learn more?