They say massage before flu treatment is essential (7)
I believe the answer is:
needful
'essential' is the definition.
(thesaurus)
'they say massage before flu treatment' is the wordplay.
'they say' shows a homophone (sound like).
'massage' becomes 'knead' ('knead' can be a synonym of 'massage').
'before' says to put letters next to each other.
'flu treatment' becomes 'ful' (I can't explain this - if you can you should give a lot more credence to this answer).
'knead' is a homophone of 'need'.
'need'+'ful'='NEEDFUL'
'is' is the link.
(Other definitions for needful that I've seen before include "Nude elf (anag.)" , "Required" , "Indispensable" , "poor" , "Poverty-stricken" .)