Clear writer with one's signature at the end (9)
I believe the answer is:
authorise
'clear' is the definition.
('authorise' can be a synonym of 'clear')
'writer with one's signature at the end' is the wordplay.
'writer' becomes 'author' ('author' can be a synonym of 'writer'**).
'with' is a charade indicator (letters next to each other).
'one's' becomes 'is' (roman numeral).
'at the end' suggests the final letters.
The last letter of 'signature' is 'e'.
'author'+'is'+'e'='AUTHORISE'
(Other definitions for authorise that I've seen before include "licence" , "Give power to or sanction" , "I share out (anag.)" , "Give official permission to" , "commission" .)