The length of the alliance (6)
I believe the answer is:
league
'the alliance' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for league that I've seen before include "Association for a common purpose" , "Former measure of distance, usually about three miles" , "Old measure of distance, about 3 miles" , "Association of states or sports teams" , "Group of teams competing over a period" .)