What does the cryptic crossword clue "Tate Modern's 9:23 (11)" mean?

The wordplay uses an anagram. anagram of TATE ("Tate") + MODERNS ("Modern's 9:23") = DEMONSTRATE The answer is DEMONSTRATE.

Tate Modern's 9:23 (11)

Guardian Cryptic #29120 | 12 Down | 2023-07-12 flower