Rebel sounds quiet between these walls (8)
I believe the answer is:
mutineer
'rebel' is the definition.
(synonyms)
I cannot understand how the rest of the clue works.
(Other definitions for mutineer that I've seen before include "One in open rebellion" , "Maybe Christian" , "Eg, one of Bounty crew" , "One openly rebelling" , "Openly rebellious person" .)