Settled outcome, concealing last of arsenic in cake (9)
I believe the answer is:
ensconced
'settled' is the definition.
('ensconce' can be a synonym of 'settle')
'outcome concealing last of arsenic in cake' is the wordplay.
'outcome' becomes 'end' (I've seen this in other clues).
'concealing' means one lot of letters goes inside another.
'last of arsenic in cake' becomes 'sconce' (I can't justify this - if you can you should believe this answer much more).
'end' going around 'sconce' is 'ENSCONCED'.
Can you help me to learn more?
(Other definitions for ensconced that I've seen before include "in safe seat" , "Settled comfortably" , "Installed" , "Safely established" .)