Liable to be lying down (5)
I believe the answer is:
prone
This clue is a double definition.
'liable' is the first definition.
(both can mean having a tendency for something)
'be lying down' is the second definition.
(I've seen this in another clue)
'to' is the link.
(Other definitions for prone that I've seen before include "Liable (to suffer) - lying down" , "Placed face down" , "Lying face down - inclined" , "Lying face downwards" , "Recumbent, likely" .)