Not feeling well (3)
(Other definitions for ill that I've seen before include "Misfortune" , "Unwell, sick" , "Wicked" , "Valetudinarian" , "Indisposed" .)
'not feeling well' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ailing that I've seen before include "Troubled in mind or body" , "Infertile" , "Ill" , "Unwell, in poor condition" , "not doing very well" .)