This not kosher? Don't believe it! (4,3)

I believe the answer is:

pork pie

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'don't believe it' is the definition.
(cockney rhyming slang for a lie)

I cannot understand how the remainder of the clue works.

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(Other definitions for pork pie that I've seen before include "swallowed by gull" , "Cooked meat in pastry" , "going over one's head?" , "Eat this < > hat" , "that can be eaten!" .)

I've seen this clue in The Sun.

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