Class length (6)
I believe the answer is:
league
This might be a double definition.
'class' is the first definition.
(I've seen this before)
'length' is the second definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
Can you help me to learn more?
(Other definitions for league that I've seen before include "Combination of groups for a purpose, sports clubs perhaps" , "Association - old measure of distance" , "Old measure of distance, about 3 miles" , "Association of states or sports teams" , "Competing group of clubs" .)