The most common and most likely answer to this crossword clue is the 8 letter word MATESHIP
I've seen this crossword clue in The Guardian
M
A
T
E
S
H
I
P
More clues leading to the result MATESHIP
Buddy explains!

The straight part
We know that "Attachment from down under" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case attachment from down under = mateship.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 MATESHIP mean?
MATESHIP noun- The post of mate on a ship; a posting as mate.
- (whaling) A type of contract between ships to cooperate and share the proceeds of an expedition.
- Fellowship; companionship.
- Friendship, particularly between men, such as develops in shared adversity; solidarity.
- A relationship based on mating.