Thanks to a book being unfinished, it's banned (5)
I believe the answer is:
taboo
'banned' is the definition.
(I've seen this before)
'thanks to a book being unfinished' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'to' is a charade indicator (letters next to each other).
'being unfinished' means to remove the last letter.
'book' with its final letter taken off is 'boo'.
'ta'+'boo'='TABOO'
'it's' acts as a link.
(Other definitions for taboo that I've seen before include "Strictly forbidden" , "Prohibited by social custom" , "Social prohibition" , "Forbidden for social reasons" , "Not to be mentioned" .)