Rule adopted by a very old Aussie PM (4)
I believe the answer is:
arvo
'aussie pm' is the definition.
(Australian term for the afternoon - PM on a clock)
'rule adopted by a very old' is the wordplay.
'rule' becomes 'r' (abbreviation for rule).
'adopted by' means one lot of letters goes inside another (I've seen this in other clues).
'very' becomes 'v' (common abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'a'+'v'='av'
'r' inserted inside 'av' is 'arv'.
'arv'+'o'='ARVO'
(Other definitions for arvo that I've seen before include "Australian PM" , "afternoon in Sydney" , "PM (Aus.); --- Part, composer" , "Aussie PM" , "PM in Canberra" .)