Most of page author used for signature (9)
I believe the answer is:
autograph
'for signature' is the definition.
(autograph is a kind of signature)
'most of page author used' is the wordplay.
'most of' means to remove the last letter.
'used' indicates anagramming the letters.
'page' with its last letter taken off is 'pag'.
'pag'+'author'='pagauthor'
'pagauthor' with letters rearranged gives 'AUTOGRAPH'.
(Other definitions for autograph that I've seen before include "Sign" , "a collector's item perhaps" , "(Celebrity) signature" , "A signature given as souvenir" , "Famous person's signature" .)