Union J has nothing against One Direction (4)
I believe the answer is:
join
'union' is the definition.
('join' can be a synonym of 'union')
'j has nothing against one direction' is the wordplay.
'has nothing' becomes 'o' (I've seen this in other clues).
'against' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral**).
'direction' becomes 'n' (N is an example).
'j'+'o'+'i'+'n'='JOIN'
(Other definitions for join that I've seen before include "Attach" , "Couple" , "make ends meet" , "ally" , "Amalgamate" .)