Ailing, unhealthy (6)
I believe the answer is:
sickly
I believe this is a double definition.
'ailing' is the first definition.
(I know that ailing can be written as sickly)
'unhealthy' is the second definition.
(similar in meaning)
(Other definitions for sickly that I've seen before include "Looking unwell" , "Ailing, unhealthy" , "Given often to poor health" , "Feeble, mawkish" , "Prone to illness" .)