Debility (8)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
weakness
'debility' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for weakness that I've seen before include "Ill-defended spot - predilection" , "Defect" , "Infirmity" , "Achilles heel" , "Frailty - flaw" .)