A less than immaculate garment (3)

I believe the answer is:

mac

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

'garment' is the definition.
(mac is a kind of garment)

'a less than immaculate' is the wordplay.
'MAC' is hidden within the letters.
But, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for mac that I've seen before include "A rainproof" , "Waterproof coat (colloq.)" , "Waterproof raincoat, in short" , "PC" , "A Gaelic son, often in name" .)

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