Set upon one's fiancee (8)
I believe the answer is:
intended
'fiancee' is the definition.
(intended can mean someone's fiancee)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for intended that I've seen before include "Deliberate" , "wanted from the outset" , "Fiance (e) - on purpose" , "Meant - fiancee" , "Had in mind to" .)