The most common and most likely answer to this crossword clue is the 6 letter word FLY-IN
I've seen this crossword clue in The Guardian
F
L
Y
-
I
N
Buddy explains!

The straight part
We know that "Gathering of the air" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case gathering of the air = fly-in.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 FLY-IN mean?
FLY-IN noun- An informal gathering of private pilots and their aircraft at a prearranged airfield.
- (roofing) A method of application for roll materials by which the dry sheet is set into the bitumen or adhesive applied to the roof surface.