The most common and most likely answer to this crossword clue is the 9 letter word MONOPLANE
I've seen this crossword clue in The Times
M
O
N
O
P
L
A
N
E
Buddy explains!

The straight part
We know that "aircraft" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case aircraft = monoplane.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 MONOPLANE mean?
MONOPLANE noun- An airplane that has a single pair of wings
- To fly in a monoplane.