Start pitying errant niece as nun craves expiatory atonement (7)
I believe the answer is:
penance
'atonement' is the definition.
(thesaurus)
'start pitying errant niece as nun craves expiatory' is the wordplay.
'start' says to take the initial letters.
The first letters of 'pitying errant niece as nun craves expiatory' is 'PENANCE'.
(Other definitions for penance that I've seen before include "Self-imposed punishment" , "Act performed in sorrow for sin" , "Voluntary punishment to atone for sin" , "Self-punishment for sin" , "Act of atonement" .)