Pain in the lower back (7)
I believe the answer is:
lumbago
'pain in the lower back' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lumbago that I've seen before include "Lower back ache" , "Back affliction" , "Rheumatism of lower back" , "Rheumatic pain in muscles of lower back" , "Rheumatic pain in the small of the back" .)