Slender Zulus separately infiltrating drunken soiree (4,4)

I believe the answer is:

size zero

'slender' is the definition.
(small clothing size)

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

I've seen this clue in The Times.

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.