It's star-studded party given by two unknowns (6)
I believe the answer is:
galaxy
'it's star-studded' is the definition.
'party given by two unknowns' is the wordplay.
'two unknowns' means two replacements for 'unknown'.
'party' becomes 'gala' (I've seen this before**).
'given' means one lot of letters go next to another.
'by' says to put letters next to each other.
'unknown' becomes 'x' (algebra).
'unknown' becomes 'y' (algebra).
'gala'+'x'+'y'='GALAXY'
(Other definitions for galaxy that I've seen before include "The Milky Way, for example" , "Assemblage of stars" , "Huge collection of stars" , "many stars" , "e.g. The Milky Way" .)