Sign a letter from the Pope (4)

I believe the answer is:

bull

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'sign' is the definition.
(I know that Bull is a sign)

I can't explain the rest of the clue.

(Other definitions for bull that I've seen before include "fifty on board" , "Tripe" , "Male whale" , "Bovine charger?" , "Cow's mate" .)

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

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