The mortification of writer on treatment of acne (7)
I believe the answer is:
penance
'the mortification' is the definition.
(thesaurus)
'writer on treatment of acne' is the wordplay.
'writer' becomes 'pen' (I've seen this before**).
'on' means one lot of letters goes inside another.
'treatment of' indicates an anagram.
'acne' anagrammed gives 'ance'.
'pen' put within 'ance' is 'PENANCE'.
'of' is the link.
(Other definitions for penance that I've seen before include "Expression of sorrow for sin" , "Act performed in sorrow for sin" , "Voluntary self-punishment" , "Punishment imposed for sin" , "Voluntary punishment to atone for sin" .)