Reason to stamp out cattle illness (9)
I believe the answer is:
discourse
'reason' is the definition.
(thesaurus)
'stamp out cattle illness' is the wordplay.
'stamp' becomes 'die' (I've seen this in other clues).
'out' indicates putting letters inside.
'cattle illness' becomes 'scours' (scours can mean diarrhoea in cattle).
'die' placed around 'scours' is 'DISCOURSE'.
'to' is the link.
(Other definitions for discourse that I've seen before include "Engage in conversation; debate" , "Formal discussion of a topic" , "(Formal) talk" , "verbal communication" , "Speech - conversation" .)