Garment needing a bit of machining (3)

I believe the answer is:

mac

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

I can't explain the remainder of the clue.

(Other definitions for mac that I've seen before include "A Gaelic son, often in name" , "A rainproof" , "Coat - computer" , "one used in shower?" , "Waterproof coat (colloq.)" .)

I've seen this clue in the Evening Standard.

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