Unexpected defeat of leading group (5)
I believe the answer is:
upset
'unexpected defeat' is the definition.
(I've seen this before)
'leading group' 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'
'of' is the link.
(Other definitions for upset that I've seen before include "Disturb or knock over" , "Spilt - annoyed" , "Surprise win" , "Overturn - perturbed - disordered" , "Discountenanced" .)