Books supporter’s written about King Edward, truly (9)
I believe the answer is:
'truly' is the definition.
'books supporter's written about king edward' is the wordplay.
'books' becomes 'lit' (abbreviation for literature).
'supporter' becomes 'ally' (an ally is a friend or supporter).
'written about' indicates putting letters inside.
'king edward' becomes 'er' (abbreviation for Eduardus Rex).
'litally' enclosing 'er' is 'LITERALLY'.
(Other definitions for literally that I've seen before include "Not figuratively" , "Absolutely" , "Without exaggeration" , "According to the letter, exactly" , "precisely as written" .)