It's rumoured head teacher will mention casual greeting received (7)
I believe the answer is:
theysay
'it's rumoured' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'head teacher will mention casual greeting received' is the wordplay.
'head' indicates taking the first letters.
'will' means one lot of letters go next to another (I've seen this in other clues).
'mention' becomes 'say' (I've seen this in another clue).
'casual greeting' becomes 'hey' (I've seen this in another clue).
'received' means one lot of letters goes inside another.
The first letter of 'teacher' is 't'.
't'+'say'='tsay'
'tsay' enclosing 'hey' is 'THEYSAY'.
Can you help me to learn more?