Fight a spell of illness (4)
I believe the answer is:
bout
I believe this is a double definition.
'fight' is the first definition.
(as in a bout in boxing)
'a spell of illness' is the second definition.
(eg a bout of flu)
(Other definitions for bout that I've seen before include "Wrestling contest" , "wrestling match" , "Fight; instance of illness" , "Short period of intense activity" , "Spell - boxing match" .)