About three miles for union (6)
I believe the answer is:
league
I believe this clue is a double definition.
'about three miles' is the first definition.
(I've seen this in another clue)
'union' is the second definition.
(I know that league is a type of union)
'for' acts as a link.
(Other definitions for league that I've seen before include "Group in football" , "Federation" , "See 11" , "Association for a common purpose" , "band together" .)