Relating to the lower part of the back (6)
I believe the answer is:
lumbar
'relating to the lower part of the back' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lumbar that I've seen before include "Relating to the lower part of the back" , "Related to lower back" , "Relating to the lower backR" , "Relating to the back between lowest ribs and pelvis" , "Relating to the lower spine" .)