Successfully making seven 14? (5,4)
I believe the answer is:
grand slam
'14?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for grand slam that I've seen before include "Winning all tricks in a hand of 1 across" , "Winning of all of a group of tournaments in tennis" , "Winning of all tricks (or major tournaments)" , "Winning of all relevant contests" , "sporting triumph" .)