A wife's flu treatment is dire (5)
I believe the answer is:
awful
'is dire' is the definition.
(synonyms)
'a wife's flu treatment' is the wordplay.
'wife' becomes 'w' (abbreviation).
'treatment' indicates an anagram (letters treated or processed differently).
'flu' anagrammed gives 'ful'.
'a'+'w'+'ful'='AWFUL'
(Other definitions for awful that I've seen before include "Very poor" , "shocking" , "Dreadful and dire" , "Horrific" , "Atrocious, dire" .)