Butcher regarding famous horse being rejected (8)
I believe the answer is:
murderer
'butcher' is the definition.
(thesaurus)
'regarding famous horse being rejected' is the wordplay.
'regarding' becomes 're' (re can mean regarding, concerning).
'famous horse' becomes 'Red Rum' (three time winner of the Grand National).
'being rejected' shows that the letters should be reversed in order.
're'+'redrum'='reredrum'
'reredrum' written backwards gives 'MURDERER'.
(Other definitions for murderer that I've seen before include "One commits homicide" , "Assassin" , "Deliberate unlawful killer" , "Criminal killer" , "hit man" .)