A garment — absence of it exposes old king (4)

I believe the answer is:

ahab

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'old king' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for ahab that I've seen before include "Fictional whaler" , "Melville's whale chaser" , "Melville's Captain" , "Captain -, pursuer of Moby Dick" , "Spring bulbs" .)

I've seen this clue in The Times.

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