What does the cryptic crossword clue "Buff host with no clothing is what could alarm Sky viewer (4)" mean?

The wordplay uses a deletion. UF (deletion) + (deletion) + OS (deletion) + (deletion) = UFOS The answer is UFOS.

Buff host with no clothing is what could alarm Sky viewer (4)

Guardian Cryptic #29580 | 21 Across | 2025-01-01 flower