Watertight chamber is thus enclosed in metal (7)
I believe the answer is:
caisson
'watertight chamber' is the definition.
(I've seen this before)
'is thus enclosed in metal' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'enclosed in' is an insertion indicator.
'metal' becomes 'can' (I can't justify this - if you can you should believe this answer much more).
'is'+'so'='isso'
'isso' inserted into 'can' is 'CAISSON'.
Can you help me to learn more?
(Other definitions for caisson that I've seen before include "watertight chamber" , "Structure excluding water from construction" , "Water-excluding case" , "So I scan the ammunition wagon" .)