Unexpected results from leading groups (6)
I believe the answer is:
upsets
'unexpected results' is the definition.
(I've seen this before)
'leading groups' is the wordplay.
'leading' becomes 'up' ('up' can be similar in meaning to 'leading').
'groups' becomes 'sets' (set is a kind of group).
'up'+'sets'='UPSETS'
'from' is the link.
(Other definitions for upsets that I've seen before include "Makes unhappy or worried" , "Disturbs or knocks over" , "Unexpected results" , "Disapppointments" , "Hassles" .)