Simple when you think about it

After East opens 3♦, you become the declarer in 6♥ on this layout (you are vulnerable, the opponents are not):

                ♠ K Q J 10 3
                ♥ 7 2
                ♦ 6 5 4
                ♣ K 10 9
♠ A 9 8 7 5                   ♠ 6 4 3
♥ 9 5 4                       ♥ 8
♦ 2                           ♦ K Q J 10 8 7
♣ 8 7 5 3                     ♣ J 6 4
                ♠ ---
                ♥ A K Q J 10 6 3
                ♦ A 9 3
                ♣ A Q 2

The ♦2 lead is an obvious singleton and West is most likely to hold the ♠A. Let's say you draw three rounds of trumps, discarding a diamond from dummy. There are no squeezes that I see, so to make the contract, you will have to set up the spades by conceding the ace, then get back to dummy to enjoy them.

One way to do this would be to lead a low club and put in the 10 from dummy. That would be a failure on this lie of the cards -- East would win and cash two diamond tricks. Besides that, an alert West could insert the ♣J when you lead low towards dummy to foil this plan.

The answer is to lead the ♣Q and overtake with dummy's ♣K. When you lead the ♠K and discard a diamond, West can win but what then? A spade return can be won in dummy, and a club exit will promote the ♣10 9 into an entry regardless of who holds the ♣J.