The issue with his novel - it's slightly immature (8)
I believe the answer is:
youngish
'slightly immature' is the definition.
'youngish' can be an answer for 'immature' ('youngish' can be similar in meaning to 'immature'). I'm not certain of the 'slightly' bit.
'the issue with his novel' is the wordplay.
'the issue' becomes 'young' (I've seen this before**).
'with' means one lot of letters go next to another.
'novel' indicates an anagram.
'his' anagrammed gives 'ish'.
'young'+'ish'='YOUNGISH'
'it's' is the link.
Can you help me to learn more?
(Other definitions for youngish that I've seen before include "Fairly youthful" , "Rather immature" , "Not very old" .)