Pack zero pastry cases (5)
I believe the answer is:
tarot
'pack' is the definition.
(type of card pack)
'zero pastry cases' is the wordplay.
'zero' becomes 'o'.
'pastry' becomes 'tart' (tart is a kind of pastry).
'cases' is an insertion indicator.
'o' put within 'tart' is 'TAROT'.
(Other definitions for tarot that I've seen before include "Set of cards used by fortune-tellers" , "Mystic cards" , "Sort of cards" , "Pack of cards used in fortune-telling" , "Cards used for fortune telling" .)