Defeat prearranged after leading (5)
I believe the answer is:
upset
'defeat' is the definition.
(upsetting is a kind of defeating)
'prearranged after leading' is the wordplay.
'prearranged' becomes 'set' (I can't justify this - if you can you should believe this answer much more).
'after' says to put letters next to each other.
'leading' becomes 'up' (up can mean winning a sports match).
'set' after 'up' is 'UPSET'.
Can you help me to learn more?
(Other definitions for upset that I've seen before include "thrown?" , "overcome" , "Out of sorts" , "'Disturbed, agitated (5)'" , "Disrupted; knock over" .)