Clue: Group inch aboard craft, an old chopper (10)
Answer: GUILLOTINE
Explanation: Definition: an old chopper
 [ LOT(a group of similar items) + IN(abbrev. for "inch", the unit of length) ] **contained in**(aboard) GUILE(craft/cunning).
Defn: …, in this instance, used to chop off people's heads, famously used during the French Revolution.