Books supporter’s written about King Edward, truly (9)
I believe the answer is:
literally
'truly' is the definition.
(thesaurus)
'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).
'lit'+'ally'='litally'
'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" .)