... beat leaders of Champions League (United & Barcelona) (4)
I believe the answer is:
club
'beat' is the definition.
(I've seen this before)
'leaders of champions league united barcelona' is the wordplay.
'leaders of' suggests taking the first letters.
The initial letters of 'champions league united barcelona' is 'CLUB'.
(Other definitions for club that I've seen before include "Association of people with similar interests" , "Stout stick, card suit and association" , "Place to meet one's friends with a weapon" , "Association for people with common interest" , "wood, perhaps" .)