Crossword Genius

Spooner's pastry shortage? This isn't to be trusted (4,2,4)

Ross

I believe the answer is:

pack of lies

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'this isn't to be trusted' is the definition.
(a pack of lies can't be believed in)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for pack of lies that I've seen is " Web of untruths".)

I've seen this clue in The Guardian.
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