Murderer outside second-class compartment (5)
I believe the answer is:
cabin
'compartment' is the definition.
(I know that cabin is a type of compartment)
'murderer outside second-class' is the wordplay.
'murderer' becomes 'cain' (Biblical figure).
'outside' indicates putting letters inside (some letters go outside others).
'second class' becomes 'b'.
'cain' enclosing 'b' is 'CABIN'.
(Other definitions for cabin that I've seen before include "Small room on a ship" , "Railway signal box" , "class at sea?" , "Marine digs" , "Small room or hut" .)