Disdain a treatment for corns (5)
I believe the answer is:
scorn
'disdain' is the definition.
('scorn' can be a synonym of 'disdain')
'a treatment for corns' is the wordplay.
'a treatment for' is an anagram indicator (I've seen this in other clues).
'corns' is an anagram of 'SCORN'.
(Other definitions for scorn that I've seen before include "Spurn" , "Disdain or derision" , "Open disrespect" , "Reject contemptuously" , "Open contempt, disdain" .)