Loner largely over tummy trouble (5)

I believe the answer is:

ulcer

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'tummy trouble' is the definition.
(I've seen this in another clue)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for ulcer that I've seen before include "Sore point" , "Stomach or mouth sore" , "Continuing source of pain" , "Cruel form of stomach trouble?" , "Cruel sort of body sore" .)

I've seen this clue in the Sydney Morning Herald.

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