Winning group's surprise result (5)
I believe the answer is:
upset
'surprise result' is the definition.
(I've seen this before)
'winning group's' is the wordplay.
'winning' becomes 'up' (eg being up at half time in football).
'group' becomes 'set' (both can mean a collection).
'up'+'set'='UPSET'
(Other definitions for upset that I've seen before include "Discountenanced" , "Disturb(ance)" , "Aggrieved" , "Surprise result" , "Overturn; unhappy" .)