Leading group's surprise result (5)
I believe the answer is:
upset
'surprise result' is the definition.
(I've seen this in another clue)
'leading group's' is the wordplay.
'leading' becomes 'up' (up can mean leading in a game).
'group' becomes 'set' (both can mean a collection).
'up'+'set'='UPSET'
(Other definitions for upset that I've seen before include "dismay" , "Spilt - annoyed" , "Overturn; unhappy" , "Make unhappy; disrupt" , "Disturb or knock over" .)