Murderer, dangerous ultimately, therefore kept in special chamber (7)
I believe the answer is:
'special chamber' is the definition.
(watertight chamber used in construction)
'murderer dangerous ultimately therefore kept' is the wordplay.
'murderer' becomes 'cain' (Cain who killed Abel).
'ultimately' indicates one should take the final letters.
'therefore' becomes 'so' ('so' can mean 'consequently' or 'therefore').
'kept' means one lot of letters goes inside another.
The last letter of 'dangerous' is 's'.
'cain' placed around 'sso' is 'CAISSON'.
'in' acts as a link.
(Other definitions for caisson that I've seen before include "Structure excluding water from construction" , "watertight chamber" , "Water-excluding case" , "So I scan the ammunition wagon" .)