The most common and most likely answer to this crossword clue is the 6 letter word BOOMER
I've seen this crossword clue in The Times
B
O
O
M
E
R
Buddy explains!
The straight part
We know that "Big Aussie" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case big aussie = boomer.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.
What does BOOMER mean?
BOOMER noun- A person born in the postwar years (generally considered in the United States and other Allied countries as between 1946 and the early 1960s), when there was a marked increase in birthrates throughout the Western world following the return of servicemen at the end of World War II.
- An adult male kangaroo.
- (by extension) A person who does not know how to utilize new technologies well.
- Something that makes a booming sound.
- (mainly 1920–1930) A transient worker who would move from boom town to boom town in search of temporary work.
- A device used to bind or tighten chain.
- A nuclear ballistic missile submarine, SSBN.
- A bittern (subfamily Botaurinae).
- A red squirrel (Tamiasciurus hudsonicus).
- A mountain beaver (Aplodontia rufa).