Instinctive reversal of leading vessel (3)
I believe the answer is:
gut
'instinctive' is the definition.
(I've seen this in other clues)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for gut that I've seen before include "Material for violins" , "Intestine, or the stomach" , "Part of the digestive system" , "strong feelings" , "Food processor" .)