The most common and most likely answer to this crossword clue is the 5 plus 4 letter word FLARE PATH
F
L
A
R
E
P
A
T
H
Buddy explains!
The straight part
We know that "airstrip – that’s brilliant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case airstrip – that’s brilliant = flare path.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.