Prostrate (5)
(Other definitions for prone that I've seen before include "Liable (to suffer) - lying down" , "Lying face downward" , "Placed face down" , "Lying face down or likely to do something" , "Given" .)
'prostrate' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for lying that I've seen before include "Resting horizontally; fibbing" , "Flat-out" , "So one can't stand" , "Prone - untruthful" , "Untruthful - in bed?" .)