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 wordplay.
'initially' says to take the initial letters.
'by doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus. I am not sure about the 'by' bit.**).
'in the past' becomes 'ago' ('ago' can be a synonym of 'in the past').
The first letters of 'left untreated' is 'lu'.
(Other definitions for lumbago that I've seen before include "Rheumatic backache", "Complaint", "Backache", "Lower back ache", "A rheumatic pain", "Rheumatism of lower back", "Form of backache", "Rheumatic pain in the small of the back", "Pain in the lower back" and "Back affliction".)