Officer liable to plug 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 plug brother's killer' is the wordplay.
'liable' becomes 'apt' (synonyms).
'to plug' means one lot of letters goes inside another (to plug can mean to block a gap within something).
'brother's killer' becomes 'cain' (I can't explain this - if you can you should give a lot more credence to this answer).
'apt' put within 'cain' is 'CAPTAIN'.
Can you help me to learn more?
(Other definitions for captain that I've seen before include "Leader" , "Commander of a ship" , "Chief" , "Lead (a team)" , "Skipper" .)