The most common and most likely answer to this crossword clue is the 9 letter word SEMILUNAR
I've seen this crossword clue in The Guardian
S
E
M
I
L
U
N
A
R
Buddy explains!

The straight part
We know that "like the aortic valve" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case like the aortic valve = semilunar.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 SEMILUNAR mean?
SEMILUNAR noun- The lunate bone, or semilunar bone.
- Shaped like a half-moon; crescent-shaped.