Part of what one mentions is reparation (9)
I believe the answer is:
atonement
'is reparation' is the definition.
(thesaurus)
'part of what one mentions' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'ATONEMENT' is hidden within 'what one mentions'.
(Other definitions for atonement that I've seen before include "'Penance, redemption (9)'" , "needed to make amends" , "Reparation (for a wrong)" , "Compensation for a wrong" , "Expiation of sin, penance" .)