Worrying note from teacher with terrible flu (9)
I believe the answer is:
stressful
'worrying' is the definition.
(I've seen this before)
'note from teacher with terrible flu' is the wordplay.
'note from teacher' becomes 'stress' (I can't justify this - if you can you should give a lot more credence to this answer).
'with' means one lot of letters go next to another.
'terrible' is an anagram indicator.
'flu' anagrammed gives 'ful'.
'stress'+'ful'='STRESSFUL'
Can you help me to learn more?
(Other definitions for stressful that I've seen before include "tense" , "under great pressure" , "Creating tension" , "Very hard on the nerves" , "Worrying" .)