An irritating habit (4,5)
I believe the answer is:
hair shirt
'habit' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'an irritating' is the wordplay.
I cannot quite understand 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 present in the remaining letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for hair shirt that I've seen before include "Penitential garment of hirsute kind" , "Hirsute penitential garment for men" , "A penitential garment of hirsute beginnings" , "Penitent's garb" , "one's ashamed to wear it" .)