Forbidden fruit a book covers (5)
I believe the answer is:
taboo
'forbidden' is the definition.
(synonyms)
'fruit a book covers' is the wordplay.
'covers' indicates the answer is hidden within the clue (I've seen 'covered' mean this (the answer is covered up by the clue)).
'TABOO' is hidden within 'fruit a book'.
(Other definitions for taboo that I've seen before include "A ban or prohibition of superstitious kind" , "Forbidden by social custom" , "Prohibited by religion or custom" , "Ritually forbidden" , "Strictly forbidden" .)