Gala — celebration (6)
I believe the answer is:
I believe this clue is a double definition.
'gala' is the first definition.
'celebration' is the second definition.
(fiesta is a kind of celebration)
(Other definitions for fiesta that I've seen before include "Religious festival in Hispanic communities" , "Outdoor celebration" , "Religious event" , "Festival, fete" , "A Spanish public celebration" .)