Discredit wilderness guru seen cycling (5)

I believe the answer is:

smear

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

I cannot understand how the remainder of the clue works.

(Other definitions for smear that I've seen before include "Blur, smudge" , "Smother" , "Medical examiner may take this" , "sample on slide" , "Slander - tissue sample used for testing" .)

I've seen this clue in The Independent.

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