A wife treated flu that's very bad (5)
I believe the answer is:
awful
'very bad' is the definition.
(I've seen this before)
'a wife treated flu' is the wordplay.
'wife' becomes 'w' (abbreviation).
'treated' indicates an anagram.
'flu' is an anagram of 'ful'.
'a'+'w'+'ful'='AWFUL'
'that's' acts as a link.
(Other definitions for awful that I've seen before include "Appalling, terrible" , "Extremely bad or unpleasant" , "Really terrible, ghastly" , "Very poor" , "rubbish" .)