Manner adopted by Arab penitent (6)
I believe the answer is:
atoner
'penitent' is the definition.
(I've seen this in another clue)
'manner adopted by arab' is the wordplay.
'manner' becomes 'tone'.
'adopted by' is an insertion indicator (I've seen this in other clues).
'arab' becomes 'Ar.' (abbreviation in Chambers).
'tone' inserted inside 'ar' is 'ATONER'.
(Other definitions for atoner that I've seen before include "One who makes amends for wrongs done" , "Person making amends for wrongs committed" .)