Backache (7)
I believe the answer is:
lumbago
'backache' is the definition.
(I know that lumbago is a type of backache)
This is all the clue.
(Other definitions for lumbago that I've seen before include "Rheumatic backache" , "Rheumatic pain in muscles of lower back" , "Back affliction" , "Back problem" , "Maul bog badly for a rheumatic affliction" .)