Brought together (6)
I believe the answer is:
'brought together' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for united that I've seen before include "Married pair, are, like Man U!" , "Joined together -- in duet (anag)" , "Leeds, Sheffield or Manchester, perhaps" , "cooperative" , "Mark -, Russian-born US abstract painter" .)