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