It shows us the way to authorise mail (8)
I believe the answer is:
signpost
'it shows us the way' is the definition.
(a signpost gives directions)
'authorise mail' is the wordplay.
'authorise' becomes 'sign' (signing something can mean authorising it).
'mail' becomes 'post' (synonyms).
'sign'+'post'='SIGNPOST'
'to' acts as a link.
(Other definitions for signpost that I've seen before include "Indication of direction" , "pointer along the way" , "Waymark" , "indication how to progress" , "director" .)