Firm back to collect document with mistakes (9)

I believe the answer is:

erroneous

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

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

(Other definitions for erroneous that I've seen before include "Mistaken, in error" , "with mistakes" , "Incorrect, wrong" , "Mistaken - inaccurate" , "not right" .)

I've seen this clue in The Sun.

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