Killer only dear to the French (7)
'killer' is the definition.
(butcher is a kind of killer)
'only dear to the french' is the wordplay.
'only' becomes 'but' (but can mean only - eg 'life is but a dream').
'dear to the french' becomes 'cher' ('dear' in French).
(Other definitions for butcher that I've seen before include "Killer", "mess up?", "Meat supplier", "In EastEnders, Pat maybe", "Kill brutally".)