Portion of what one mentioned as reparation (9)
I believe the answer is:
atonement
'reparation' is the definition.
(thesaurus)
'portion of what one mentioned' is the wordplay.
'portion of' indicates the answer is hidden within the clue.
'ATONEMENT' can be found hidden inside 'what one mentioned'.
'as' is the link.
(Other definitions for atonement that I've seen before include "needed to make amends" , "Compensation for a wrong" , "Making good" , "'Penance, redemption (9)'" , "Reparation (for a wrong)" .)