Unhealthy looking pastry lacks right ingredient (3)

I believe the answer is:

wan

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'unhealthy looking' is the definition.
(I've seen this in another clue)

I don't understand the remainder of the clue.

(Other definitions for wan that I've seen before include "Pale and weak-looking" , "Pallid" , "Pale and looking ill" , "Faint" , "Looking pale and sickly" .)

I've seen this clue in The Irish Times.

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