Words get muddled at the end of a day when tired (6)

I believe the answer is:

drowsy

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

I cannot understand how the remainder of the clue works.

(Other definitions for drowsy that I've seen before include "Comatose" , "Ready to drop off" , "Torpid" , "Inclined to nod off" , "liable to drop off" .)

I've seen this clue in The Globe and Mail.

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