Back trouble initially left untreated by doctor in the past (7)
I believe the answer is:
Here is my best explanation:
'back trouble' is the definition.
'initially left untreated by doctor in the past' is the subsidiary indication.
'initially' indicates taking the first letters.
'by' is a charade indicator (letters next to each other).
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus**).
'in the past' becomes 'ago' ('ago' can be a synonym of 'in the past').
The initial letters of 'left untreated' is 'lu'.
(Other definitions for lumbago that I've seen before include "Backache", "Rheumatism of lower back", "Rheumatic backache", "Pain in the lower back", "Back problem", "Complaint", "Rheumatic pain in the small of the back", "Pain -- a bum log (anag)", "Rheumatic pain in muscles of lower back" and "Back affliction".)