Ailing, sick (6)
I believe the answer is:
unwell
I believe this is a double definition.
'ailing' is the first definition.
(I know that ailing can be written as unwell)
'sick' is the second definition.
(similar in meaning)
(Other definitions for unwell that I've seen before include "Poorly" , "Off-colour" , "Indisposed" , "Sickly, low" , "Ill, not in good health" .)