Leading group's defeat (5)
I believe the answer is:
upset
'defeat' is the definition.
(upsetting is a kind of defeating)
'leading group's' is the wordplay.
'leading' becomes 'up' ('up' can be similar in meaning to 'leading').
'group' becomes 'set' (set is a kind of group).
'up'+'set'='UPSET'
(Other definitions for upset that I've seen before include "Trouble" , "Overturn; unhappy" , "Knocked over; unhappy" , "Disconcerted, discomposed" , "Make unhappy; knock over" .)