Supine's opposite (5)
I believe the answer is:
prone
'supine's opposite' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for prone that I've seen before include "Liable (to suffer) - lying down" , "Lying face downward" , "Lying flat" , "Showing a tendency" , "Prostrate" .)