It makes one itch to atone for one's misbehaviour (4,5)
I believe the answer is:
hair shirt
'misbehaviour' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'it makes one itch to atone for one's' is the wordplay.
I cannot quite understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
'one' could be 'i' (Roman numeral) and 'i' is found within the answer.
'for one' could be 'i' (I've seen this in other clues) and 'i' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for hair shirt that I've seen before include "A penitential garment of hirsute beginnings" , "Something worn by penitents" , "Penitential garment of hirsute kind" , "article for an ascetic" , "Coarse hirsute garment worn over skin as a penance" .)