Fall out of party dress (4)
I believe the answer is:
gown
'dress' is the definition.
(gown is a kind of dress)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for gown that I've seen before include "Academic attire" , "Formal robe" , "Loose dress or robe" , "Dress for formal occasions" , "Ball dress" .)