Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
make-believe [ˈmeɪkbɪliːv] n фанта́зии fpl
a world of make-believe мир фанта́зий
it's just make-believe э́то - про́сто фанта́зия
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.