It shows a way to authenticate mail (8)

I believe the answer is:

signpost

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'it shows a way' is the definition.
The definition suggests a singular noun which matches the answer.

'authenticate mail' is the wordplay.
'authenticate' becomes 'sign' (I've seen this in another clue).
'mail' becomes 'post' (synonyms).
'sign'+'post'='SIGNPOST'

'to' acts as a link.

Can you help me to learn more?

(Other definitions for signpost that I've seen before include "this will give you directions" , "Postings (anag.)" , "indication how to progress" , "Direction pointer" , "Indication of direction" .)

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

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