Excellent soldier leaves coat (3)

I believe the answer is:

mac

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

I don't understand the rest of the clue.

(Other definitions for mac that I've seen before include "Waterproof raincoat, in short" , "A rainproof" , "Raincoat (colloq.)" , "Overcoat" , "one used in shower?" .)

I've seen this clue in The Guardian.

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