Graduate writes about the awful mogul's back pain (7)

I believe the answer is:

lumbago

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'back pain' is the definition.
(I've seen this in another clue)

I don't understand how the rest of the clue works.

(Other definitions for lumbago that I've seen before include "Form of backache" , "Gaul mob (anag.)" , "Rheumatic pain in the small of the back" , "Painful condition" , "Rheumatic pain in muscles of lower back" .)

I've seen this clue in The Irish Independent.

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