What does the cryptic crossword clue "After excluding two unknowns, group of stars party (4)" mean?

The definition is "party". The wordplay uses a deletion. GALA (deletion) = GALA; definition: "party" The answer is GALA.

After excluding two unknowns, group of stars party (4)

Guardian Cryptic #29258 | 1 Down | 2023-12-20 flower