Letters sent to public figures from admirers (3,4)
I believe the answer is:
fan mail
'letters sent to public figures from admirers' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for fan mail that I've seen before include "Letters sent to public figures from admirers" , "Correspondence from admirers, as to a pop star" , "Letters from a supporter" , "Admiring post" , "Letters from devotees (including a newspaper)" .)