What does the cryptic crossword clue "Teacher who almost messed up old greeting (4,5)" mean?

The wordplay uses an anagram. anagram of TEACHER (synonym="TEACHER") + WH (synonym="WH") = WHATCHEER The answer is WHATCHEER.

Teacher who almost messed up old greeting (4,5)

Guardian Cryptic #29361 | 15 Across | 2024-04-19 flower