Happy not working, gripped by decent novel (9)
I believe the answer is:
Here is my best explanation:
'happy' is the definition.
('contented' is associated in meaning with 'happy')
'not working gripped by decent novel' is the wordplay.
'working' indicates anagramming the letters.
'gripped by' means one lot of letters goes inside another.
'novel' indicates anagramming the letters.
'not' is an anagram of 'ont'.
'decent' with letters rearranged gives 'cented'.
'ont' inserted inside 'cented' is 'CONTENTED'.