Teacher who almost messed up old greeting (4,5)
I believe the answer is:
what cheer
'up old greeting' is the definition.
The definition suggests a singular noun which matches the answer.
'teacher who almost messed' is the wordplay.
'almost' means to remove the last letter (almost the whole word is kept).
'messed' is an anagram indicator (letters get messed up).
'teacher'+'who'='teacherwho'
'teacherwho' with its final letter taken away is 'teacherwh'.
'teacherwh' with letters rearranged gives 'WHAT CHEER?'.
Can you help me to learn more?
(Another definition for what cheer that I've seen is " welcome".)