Demanding emphasis is on flu treatment (9)
I believe the answer is:
stressful
'demanding' is the definition.
'emphasis is on flu treatment' is the wordplay.
'emphasis' becomes 'stress' (synonyms).
'is on' means one lot of letters goes inside another.
'treatment' indicates an anagram (the letters must be processed or treated).
'flu' is an anagram of 'ful'.
'stress' placed within 'ful' is 'STRESSFUL'.
(Other definitions for stressful that I've seen before include "Creating tension" , "Causing mental tension" , "tense" , "under great pressure" , "Worrying" .)