The most common and most likely answer to this crossword clue is the 7 letter word LUMBAGO
I've seen this crossword clue in Independent
L
U
M
B
A
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O
More clues leading to the result LUMBAGO
- Posh doctor engaged by prisoner over back pain
- Delay arresting university doctor over complaint
- Backache? Penny must leave plant
- Album playing on journey is a pain
- Stack Beano at first with earlier back issue
- Prisoner to engage posh doctor over complaint
- Suggestion of doubt after learner driver gets to talk about old pain in back
- Business graduate stuck in Galician city – what a pain!
- Rock album attempt is a pain
- Pain cutting ear, then doctor back
Buddy explains!

The straight part
We know that "pain in back" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pain in back = lumbago.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does LUMBAGO mean?
LUMBAGO noun- Backache of the lumbar region or lower back, which can be caused by muscle strain or a slipped disk.