Characters imprisoned in dismal tower (4)

I believe the answer is:

loom

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'tower' is the definition.
('loom' can be a synonym of 'tower')

I cannot understand how the rest of the clue works.

(Other definitions for loom that I've seen before include "Hang menacingly (over)" , "Menace - weaving device" , "tower" , "Textile machine; come into sight" , "Come into view" .)

I've seen this clue in The Independent.

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