Religious festival (6)
(Other definitions for fiesta that I've seen before include "Continental knees-up" , "public event abroad" , "Public whoopee in Spain" , "Latin American celebration" , "Public celebration in Spain" .)
'religious festival' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for easter that I've seen before include "Resurrection festival" , "Card, egg or Island" , "Christian celebration of the Resurrection" , "Christian festival of celebration" , "Egg-rolling time" .)