Australian adopts the ancient town crier’s call (4)
I believe the answer is:
oyez
'town crier's call' is the definition.
(I've seen this before)
'australian adopts the ancient' is the wordplay.
'australian' becomes 'Oz' (nickname for Australia).
'adopts' indicates putting letters inside.
'the ancient' becomes 'ye' (resembles a historical spelling of 'the').
'oz' placed around 'ye' is 'OYEZ'.
(Other definitions for oyez that I've seen before include "Town crier's 'Attention!'" , "Call of hte old Town Crier" , "Call of public crier" , "Hear this" , "Town crier's shout" .)