Sorry underwear (4,5)

I believe the answer is:

hair shirt

I'm a little stuck... Click here to teach me more about this clue!

'sorry underwear' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's a link between them I don't understand?

This is all the clue.

Can you help me to learn more?

(Other definitions for hair shirt that I've seen before include "uncomfortable clothing" , "Austere garment" , "Penitent's garb" , "A penitential garment of hirsute beginnings" , "Penitential garment of hirsute kind" .)

I've seen this clue in The Globe and Mail.

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