An irritating reminder of past misdemeanours (4,5)
I believe the answer is:
hair shirt
'reminder of past misdemeanours' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'an irritating' is the wordplay.
I cannot quite see how this works, but
'an' could be 'i' (Roman numeral for one) and 'i' is found in the answer.
'irritating' could be 'harsh' (I've seen this in another clue) and 'harsh' is found within the remaining letters.
The remaining letters 'irt' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for hair shirt that I've seen before include "Hirsute penitential garment for men" , "one's ashamed to wear it" , "article for an ascetic" , "Penitential hirsute garment for upper part of man's body" , "A penitential garment of hirsute beginnings" .)