Black-tie bash (4)
I believe the answer is:
gala
'black-tie bash' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gala that I've seen before include "Festive occasion - variety of apple" , "Carnival, celebration" , "Prod with the elbow" , "Swimming competition" , "A show of charity" .)