Characters imprisoned in dismal tower (4)
I believe the answer is:
loom
'tower' is the definition.
('loom' can be a synonym of 'tower')
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(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" .)