Writer with expert accepting new self-punishment (7)
I believe the answer is:
penance
'self-punishment' is the definition.
(I've seen this before)
'writer with expert accepting new' is the wordplay.
'writer with' becomes 'pen' (I've seen this in other clues).
'expert' becomes 'ace' (ace can informally mean very good).
'accepting' means one lot of letters goes inside another (inserted letters are accepted).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'ace' going around 'n' is 'ance'.
'pen'+'ance'='PENANCE'
(Other definitions for penance that I've seen before include "Punishment for wrongdoing, by religion perhaps" , "Punishment imposed for sin" , "Remorse" , "Punishment inflicted on oneself" , "sacrament" .)