What does the cryptic crossword clue "Rival outlet collapsed — it's not to be seen (11)" mean?

The definition is "it’s not to be seen". The wordplay uses an anagram. anagram of RIVAL OUTLET = ULTRAVIOLET The answer is ULTRAVIOLET.

Rival outlet collapsed — it's not to be seen (11)

Guardian Cryptic #28992 | 8 Down | 2023-02-13 flower