The issue with his novel - it's juvenile (8)
I believe the answer is:
childish
'juvenile' is the definition.
(thesaurus)
'the issue with his novel' is the wordplay.
'the issue' becomes 'child' (I've seen this before**).
'with' means one lot of letters go next to another.
'novel' is an anagram indicator.
'his' with letters rearranged gives 'ish'.
'child'+'ish'='CHILDISH'
'it's' acts as a link.
(Other definitions for childish that I've seen before include "Juvenile" , "Puerile" , "Silly, immature" .)