Scold severely (9)
'scold severely' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for chastise that I've seen before include "Beat" , "Correct!" , "Inflict physical corrective treatment" , "Teach Sis to scold" , "Reprimand, punish" .)
(Other definitions for castigate that I've seen before include "Rebuke severely" , "Punish" , "Slap down" , "Severely criticise" , "Berate" .)