Girl clutches leg, lying on back (6)
I believe the answer is:
'lying on back' is the definition.
(I've seen this before)
'girl clutches leg' is the wordplay.
'girl' becomes 'Sue' (girl's name).
'clutches' is an insertion indicator.
'leg' becomes 'pin' (pin is a kind of leg).
'sue' placed around 'pin' is 'SUPINE'.
(Other definitions for supine that I've seen before include "Lying on one's back - passive" , "Laid-back" , "Prostrate" , "Slow in acting" , "Recumbent" .)