Sack from the French city of yore (7)
I believe the answer is:
destroy
'sack' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for destroy that I've seen before include "Get rid of" , "'Demolish, ruin (7)'" , "put to death" , "Ruin; pull down" , "Completely ruin" .)