Weapon caused the sad end of Robert the Bruce (9)
I believe the answer is:
trebuchet
'weapon' is the definition.
(I've seen this before)
'sad end of robert the bruce' is the wordplay.
'sad' is an anagram indicator.
'end of' says to take the final letters.
The last letter of 'robert' is 't'.
't'+'the'+'bruce'='tthebruce'
'tthebruce' is an anagram of 'TREBUCHET'.
'caused the' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for trebuchet that I've seen before include "Medieval siege engine" , "Medieval engine of war" , "Old weapon" , "Old slinger" , "onager?" .)