Kind of valve in the heart (6)
I believe the answer is:
aortic
'kind of valve in the heart' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for aortic that I've seen before include "Relating to the body's main artery" , "Relating to the large artery" , "of vessel" , "To do with major heart artery" .)