Posh front of mural, Buckingham Palace leaders admit, has unsatisfactory finish (4,3)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

definition: to "finish" someone i.e. to kill/murder them U="Posh" (as opposed to 'non-U' [wiki] ) plus front letter of M-[ural], both admitted into leading letters of B-[uckingham] P-[alace]; plus OFF="unsatisfactory"