Fancy dress gala is boring, bar one American (4,4)

I believe the answer is:

glad rags

I'm a little stuck... Click here to teach me more about this clue!

'fancy' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see an association between them that I don't see?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for glad rags that I've seen before include "Smart clothes (for a special occasion)" , "best suit" , "Special kit" , "Dressy clothes in happy shreds" , "Dressy clothes - happy threads" .)

I've seen this clue in The Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.