Courageous fellow with 'flu disorder (6)
I believe the answer is:
manful
'courageous' is the definition.
(I've seen this before)
'fellow with 'flu disorder' is the wordplay.
'fellow with' becomes 'man' (I've seen this in other clues).
'disorder' indicates an anagram (a disordered version of the letters).
'flu' with letters rearranged gives 'ful'.
'man'+'ful'='MANFUL'
(Other definitions for manful that I've seen before include "Virile, noble" , "Bold" , "Steadfast" , "Courageous" .)