Union measure (6)
I believe the answer is:
league
This clue is a double definition.
'union' is the first definition.
(I've seen this before)
'measure' is the second definition.
(league is a kind of measure)
(Other definitions for league that I've seen before include "some mileage?" , "Group of countries in mutual cooperation" , "Coalition" , "Old distance measure of about three miles" , "See 11" .)