Modest about one's discipline (8)
I believe the answer is:
chastise
'discipline' is the definition.
(I've seen this in another clue)
'modest about one's' is the wordplay.
'modest' becomes 'chaste' (I've seen this in another clue).
'about' indicates putting letters inside.
'one's' becomes 'i's' (Roman numeral).
'chaste' enclosing 'is' is 'CHASTISE'.
(Other definitions for chastise that I've seen before include "Whip perhaps" , "Censure severely" , "Inflict physical corrective treatment" , "Punished severely" , "Reprimand, punish" .)