What does the cryptic crossword clue "Plain tea sure to be spilt (7)" mean?

The definition is "Plain". The wordplay uses an anagram. anagram of TEA + SURE = AUSTERE; definition: "Plain" The answer is AUSTERE.

Plain tea sure to be spilt (7)

Daily Mail Cryptic #17256 | 10 Across | 2023-12-22 flower