Saintly writer is supporting rights document (9)
I believe the answer is:
charteris
'rights document' is the definition.
The definition suggests a singular noun which matches the answer.
'saintly writer is supporting' is the wordplay.
'saintly writer' becomes 'charter' (I can't justify this - if you can you should believe this answer much more).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'charter'+'is'='CHARTERIS'
Can you help me to learn more?
(Other definitions for charteris that I've seen before include "Leslie -, The Saint author" , "crime writer" .)
