Liable; lying flat (5)
I believe the answer is:
prone
I believe this is a double definition.
'liable' is the first definition.
(both can mean having a tendency for something)
'lying flat' is the second definition.
(I've seen this in another clue)
(Other definitions for prone that I've seen before include "Lying face down - inclined" , "Prostrate" , "Lying face downward" , "Given" , "Lying face down or likely to do something" .)