Hit flu treatment, being triumphant (10)
I believe the answer is:
successful
'being triumphant' is the definition.
(thesaurus)
'hit flu treatment' is the wordplay.
'hit' becomes 'success' (as in a hit song).
'treatment' indicates an anagram (the letters must be processed or treated).
'flu' is an anagram of 'ful'.
'success'+'ful'='SUCCESSFUL'
(Other definitions for successful that I've seen before include "achieving aim" , "Prosperous" , "Winning; flourishing" .)