One's brace hurt (6)
I believe the answer is:
impair
'hurt' is the definition.
(thesaurus)
'one's brace' is the wordplay.
'one's' becomes 'im' ('one is' is a formal way of saying 'I am').
'brace' becomes 'pair' (synonyms).
'im'+'pair'='IMPAIR'
(Other definitions for impair that I've seen before include "Damage or injure" , "Hurt or handicap" , "Make worse or less effective" , "Hobble" , "Harm" .)