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' indicates putting letters inside (to plug can mean to block a gap within something).
'brother's killer' becomes 'cain' (I am not sure about this - if you are sure you should believe this answer much more).
'apt' inserted inside '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" .)