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' acts as a 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" .)