Sickening (6)
'sickening' is the definition.
(I know that sickening can be written as nauseating)
This is the entire clue.
(Other definitions for nauseating that I've seen before include "arousing unpleasant feeling" , "being hard to keep down" , "Revolting" , "Loathsome" , "Stomach-turning" .)
(Other definitions for ailing that I've seen before include "not doing very well" , "Unwell, in poor condition" , "In poor health" , "Sickly, poorly" , "Infertile" .)