Lying on the back (6)
I believe the answer is:
supine
'lying on the back' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for supine that I've seen before include "Lying face upward" , "Unresisting" , "Flat on one's back" , "Laid-back" , "Lying face upwards, indolent" .)