|North leads the ♦Q against your 3NT contract. Now is the time to plan.
First count your certain tricks. 1 spade, 2 hearts, 2 diamonds and 0 clubs. Well that's five of the nine tricks you need.
When looking for a source of tricks we look to see which suit is our best suit judging on length, fit and high cards.
Once the ♣A has been knocked out you will have four club tricks. Happiness = 9 tricks in 3NTs.
Can you see any problems?
If the opposition are "savvy" when you lead clubs they will hold up the ace until the third round and you will be cut off from the last two club tricks.
You can prevent this by winning the first diamond in hand and saving the king as your entry to dummy.
You need to plan all this before you play to the first trick!