Disdain to treat corns (5)
I believe the answer is:
scorn
'disdain' is the definition.
(I know that disdain can be written as scorn)
'treat corns' is the wordplay.
'treat' is an anagram indicator.
'corns' anagrammed gives 'SCORN'.
'to' is the link.
(Other definitions for scorn that I've seen before include "Open disrespect" , "Disdain or derision" , "Open contempt, disdain" , "Contumely" , "Reject contemptuously" .)