Officer liable to stop brother's killer (7)
I believe the answer is:
captain
'officer' is the definition.
(I know that captain is a type of officer)
'liable to stop brother's killer' is the wordplay.
'liable' becomes 'apt' (synonyms).
'to stop' means one lot of letters goes inside another (stopping can mean blocking a gap).
'brother's killer' becomes 'cain' (I can't justify this - if you can you should believe this answer much more).
'apt' placed within 'cain' is 'CAPTAIN'.
(Other definitions for captain that I've seen before include "Ship's commander" , "Leader" , "Naval rank" , "eg Hornblower" , "One traditionally goes down with a sinking ship" .)