First-class Australian century against English (3)
I believe the answer is:
ace
'first-class' is the definition.
(synonyms)
'australian century against english' is the wordplay.
'australian' becomes 'A' (common abbreviation).
'century' becomes 'C' (common abbreviation).
'against' says to put letters next to each other.
'english' becomes 'E' (abbreviation).
'a'+'c'+'e'='ACE'
(Other definitions for ace that I've seen before include "Playing card with single spot" , "Service that's never returned" , "Unbeatable service" , "Tennis serve that the receiver can't reach" , "first-class" .)