At one, men finally start reparations (9)
I believe the answer is:
atonement
'reparations' is the definition.
(I've seen this before)
'at one men finally start' is the wordplay.
'finally' suggests the final letters.
The final letter of 'start' is 't'.
'at'+'one'+'men'+'t'='ATONEMENT'
(Other definitions for atonement that I've seen before include "Compensation for a wrong" , "Reparation (for a wrong)" , "needed to make amends" , "Expiation of sin, penance" , "'Penance, redemption (9)'" .)