An attack of illness is concerning (5)
I believe the answer is:
about
'is concerning' is the definition.
(about can mean concerning or relating to)
'an attack of illness' is the wordplay.
'an' becomes 'a'.
'attack of illness' becomes 'bout' (as in a bout of flu).
'a'+'bout'='ABOUT'
(Other definitions for about that I've seen before include "Approximately, around" , "On the topic of" , "Not exactly - concerning" , "In the area of or in relation to" , "Concerning or around" .)