One writing there's a universal God (6)
I believe the answer is:
author
'one writing' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'there's a universal god' is the wordplay.
'there's' means one lot of letters go next to another.
'universal' becomes 'U' (film certificate).
'god' becomes 'thor' (Thor is an example).
'a' put next to 'u' is 'au'.
'au'+'thor'='AUTHOR'
Can you help me to learn more?
(Other definitions for author that I've seen before include "eg Novelist" , "Generator, source" , "One writes" , "Producer of written work" , "He wrote the book or play" .)