The dealer sits in the South position, has dealt and turned up a queen of hearts. Picking the queen he called alone. West led the queen of spades. East followed suit with the 10. The maker(S) then trumped in with the queen.
Now the maker(S) leads the right. East follows suit with the 9 and West throws off the jack of spades.
The maker(S), knowing the left may be out against him, leads the ace of clubs. This is a safety play that could draw out remaining trump. He still holds 1 trump as a way to get back into the hand. As it turned out, the ace was good thus securing his point. Now he's thinking it's time to complete the lone.
West had different idea. Because the maker played the ace of clubs earlier in the hand the chances were good that he still held another club. (Remember, lones are often called with 3 trump plus an Ace-x off-suit). Knowing that his queen was the boss club he scraps the ace of diamonds. It was the correct move and the lone was foiled.
How would you have played this? Would you have given up the ace?