Suggestion of doubt after learner driver gets to talk about old pain in back (7)

I believe the answer is:

lumbago

I'm a little stuck... Click here to teach me more about this clue!

'pain in back' is the definition.
'lumbago' can be an answer for 'pain' (I've seen this before). I'm not sure about the 'in back' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for lumbago that I've seen before include "Back problem" , "Rheumatic pain in muscles of lower back" , "Rheumatic pain in the small of the back" , "Maul bog badly for a rheumatic affliction" , "Rheumatic backache" .)

I've seen this clue in The Independent.

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