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'.
Can you help me to learn more?
(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" .)