The most common and most likely answer to this crossword clue is the 11 letter word OUT-OF-DOOR
I've seen this crossword clue in The Times
O
U
T
-
O
F
-
D
O
O
R
Buddy explains!
The straight part
We know that "in the open air" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in the open air = out-of-door.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.