Popular publicity item for launching entertaining book is unacceptable (12)
I believe the answer is:
inadmissible
'unacceptable' is the definition.
(thesaurus)
'popular publicity item for launching entertaining book is' is the wordplay.
I cannot quite understand how this works, but
'popular' could be 'in' ('in' can mean trendy) and 'in' is located in the answer.
'publicity' could be 'ad' (short for advert) and 'ad' is present in the answer.
'book' could be 'b' (abbreviation for book) and 'b' is located in the answer.
'is' is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for inadmissible that I've seen is " Not to be tolerated".)