The most common and most likely answer to this crossword clue is the 8 plus 5 letter word MARGARET COURT
I've seen this crossword clue in Financial Times
M
A
R
G
A
R
E
T
C
O
U
R
T
Buddy explains!
The straight part
We know that "Australian tennis player" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case australian tennis player = margaret court.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.