The most common and most likely answer to this crossword clue is the 6 letter word MITRAL
I've seen this crossword clue in Independent
M
I
T
R
A
L
Buddy explains!

The straight part
We know that "of a heart valve" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case of a heart valve = mitral.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 MITRAL mean?
MITRAL adjective- Pertaining to a mitre; resembling a mitre.
- Pertaining to the mitral valve.