Major Major Major Major (5,4)
I believe the answer is:
grand slam
'major major major major' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
This is all the clue.
Can you help me to learn more?
(Other definitions for grand slam that I've seen before include "Winning of every part of contest" , "Winning all bridge tricks, or rugby games" , "ambitious contract" , "up all the top prizes" , "Winning of all 13 tricks in bridge game" .)