The most common and most likely answer to this crossword clue is the 9 letter word MUST-HAVE
I've seen this crossword clue in Financial Times
M
U
S
T
-
H
A
V
E
Buddy explains!

The straight part
We know that "Highly desirable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case highly desirable = must-have.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 MUST-HAVE mean?
MUST-HAVE noun- An item that one must own, or a feature that is essential to a service, etc.
- Essential to own.