Literature in post (7)
I believe the answer is:
letters
I believe this clue is a double definition.
'literature' is the first definition.
(I've seen this in another clue)
'post' is the second definition.
(thesaurus)
'in' acts as a link.
(Other definitions for letters that I've seen before include "...as, for example" , "Items of mail" , "A, B, C, D etc" , "They may indicate a degree" , "academic credentials" .)