It shows the way to finish correspondence (8)
I believe the answer is:
signpost
'it shows the way' is the definition.
(I've seen this before)
'finish correspondence' is the wordplay.
'finish' becomes 'sign' (as in to finish a letter by signing it).
'correspondence' becomes 'post' (letters in the post).
'sign'+'post'='SIGNPOST'
'to' is the link.
(Other definitions for signpost that I've seen before include "Direction pointer" , "this will give you directions" , "pointer along the way" , "One may help you on your way" , "Guide, indicator" .)