Writer is a universal deity (6)
I believe the answer is:
author
'writer' is the definition.
(I know that writer can be written as author)
'a universal deity' is the wordplay.
'universal' becomes 'U' (film certificate).
'deity' becomes 'Thor' (Thor is an example).
'a'+'u'+'thor'='AUTHOR'
'is' acts as a link.
(Other definitions for author that I've seen before include "Source, writer" , "Pen" , "Originate" , "Writer - initiator" , "Generator, writer" .)