The most common and most likely answer to this crossword clue is the 7 letter word WAY-OUT
I've seen this crossword clue in The Guardian
W
A
Y
-
O
U
T
More clues leading to the result WAY-OUT
Buddy explains!

The straight part
We know that "avant-garde" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case avant-garde = way-out.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 WAY-OUT mean?
WAY-OUT adjective- Unusual, unconventional or strange
- Out of the mainstream of society