Liable for flat (5)
I believe the answer is:
prone
This clue is a double definition.
'liable' is the first definition.
(thesaurus)
'flat' is the second definition.
(thesaurus)
'for' is the link.
(Other definitions for prone that I've seen before include "Lying face downwards" , "Lying flat" , "Given" , "Placed face down" , "Recumbent, likely" .)