Association footballers' union (4)
I believe the answer is:
uefa
'union' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'association footballers'' is the wordplay.
'association' becomes 'ue' (I can't justify this - if you can you should give a lot more credence to this answer).
'footballers'' becomes 'FA' (Football Association abbreviation).
'ue'+'fa'='UEFA'
Can you help me to learn more?
(Other definitions for uefa that I've seen before include "Soccer union (abbrev.)" , "Football cup" , "Organisers of Champions League" , "Sporting administrator" , "European soccer association" .)