Writer, 'a god', pens introduction to Ulysses (6)
I believe the answer is:
author
'writer' is the definition.
(I know that writer can be written as author)
'a god pens introduction to ulysses' is the wordplay.
'god' becomes 'thor' (Thor is an example).
'pens' is an insertion indicator (to pen can mean to contain or imprison).
'introduction to' indicates taking the first letters.
The initial letter of 'ulysses' is 'u'.
'a'+'thor'='athor'
'athor' placed around 'u' is 'AUTHOR'.
(Other definitions for author that I've seen before include "Source, writer" , "Generator, writer" , "He wrote the book or play" , "Creative artist" , "Writer of a book, article or document" .)