Crossword Genius

Garment that takes a bit of machining (3)

Ross

I believe the answer is:

mac

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

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for mac that I've seen before include "laptop?" , "Waterproof coat (colloq.)" , "PC" , "It may help you keep dry" , "A Gaelic son, often in name" .)

I've seen this clue in the Evening Standard.
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