Last of victims well hidden by murderer in watertight chamber (7)
I believe the answer is:
caisson
'watertight chamber' is the definition.
(I've seen this before)
'last of victims well hidden by murderer' is the wordplay.
'last of' says to take the final letters.
'well' becomes 'so' (I've seen this in other clues).
'hidden by' is an insertion indicator.
'murderer' becomes 'cain' (Cain who killed Abel).
The final letter of 'victims' is 's'.
's'+'so'='sso'
'sso' inserted into 'cain' is 'CAISSON'.
'in' acts as a link.
(Other definitions for caisson that I've seen before include "So I scan the ammunition wagon" , "Structure excluding water from construction" , "watertight chamber" , "Water-excluding case" .)