Some fantasies taboo in sleep (6)
I believe the answer is:
siesta
'sleep' is the definition.
(thesaurus)
'some fantasies taboo in' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'SIESTA' is hidden within 'fantasies taboo in'.
(Other definitions for siesta that I've seen before include "Afternoon nap in a warm climate" , "Forty winks" , "PM snoozing?" , "Spanish afternoon nap" , "Rest in the heat of the day" .)