Butcher left hare's gut in a mess (9)
I believe the answer is:
slaughter
'butcher' is the definition.
('slaughter' can be a synonym of 'butcher')
'left hare's gut in a mess' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in a mess' indicates an anagram.
'l'+'hares'+'gut'='lharesgut'
'lharesgut' is an anagram of 'SLAUGHTER'.
(Other definitions for slaughter that I've seen before include "Massacre" , "Wholesale killing" , "Kill (animals) for food" , "Butcher" , "Mass killing or carnage" .)