I've written that the rules on what to lead against no trump are subservient if you have a better lead in your hand. Generally, if your partner has bid a suit, you should lead it if you have no other lead. But look at this hand, sitting north:
West North East South
1D P 1S 2H
P P 3H P
3N P P P
What do you lead? Your hand is pretty dismal. Your first inclination is to lead your partner's suit. After all, she did overcall at the two level. However, you should think about the auction, in addition to looking at your hand. While West opened, and East responded with one spade, after South bid two hearts, West passed. Then when East reverted to West's opening suit of three diamonds, West immediately bid three no Trump. Further, your partner did not double. What does that tell you?
This auction fairly screams that West was hoping for his partner to make something akin to a reopening double (generally reopening doubles are made opening bidder, but in this instance if opener doubles the 2H bid, most good players play support doubles, so a double would promise three cards in responder's suit, spades, and would not be penalty; the only way to play 2H doubled is for opener to pass the 2H bid and for responder to protect her by doubling instead of letting it pass out). First, she passed when South overcalled 2H. Then when East bid 3H, which is Western Cue, asking partner to bid no trump if she has a stopper in opponents' suit, West confirmed a stopper by bidding 3N. That means that West is sitting behind your partner with lots of good hearts. So a heart is the last thing you want to lead in this situation, especially when partner did not double and ask you to lead her suit.
In this situation, you should revert to leading your best suit. You have five clubs headed by an honor. Since this is your best suit, and since this is a suit you would lead if your partner had not bid, and since you should not lead your partner's suit, you should lead the club five, fourth from your longest and strongest suit. In the actual hand, this lead sets the contract, and it is the only lead that would set the contract. Here's the four hand layout:
♠ K7 ♠ AQ86
♥ AQJ6 ♥ 85
♦ QT765 ♦ AKJ9
♣ T9 ♣ 643
Bidding: Both West and South took risks. South has an opening hand and fell in love with her club ace king Jack, so she risked a 2H overcall with a weak heart suit. West properly passed, hoping for a quasi penalty double. East could have made a double, because she was short in opponents' suit and could support anything partner bid, since she did have three clubs. However, she chose to bid 3H, Western Cue, a bid of opponent's suit, asking partner to bid no Trump if she has a stopper. This is risky since she does not have clubs stopped, but she probably figured if her partner has her bid, an opening hand, the odds were that she had Clubs stopped, so she took a chance. West responded to the Western Cue bid by bidding 3N. But, like her partner, she didn't have clubs stopped.
Play: From the bidding North should know not to lead a heart, leaving only a club to lead. If she leads the five of clubs South, takes the ace and King. When South returns the Jack, North overtakes with the Queen and north-south take the first five tricks, setting the contract.
In the actual hand North led the heart 4 and east-west made five, taking all but two tricks, the ace and King of clubs.
Bridge encourages flexibility in bidding and playing. There are really no rules so rigid that you can't stray from them if common sense and reason dictate. You must use your eyes, ears, and your head in playing bridge, especially in defending. If North had paid attention to the auction, used her head, and realized what the bidding meant, it would have been easy for her to find the killing lead of a club.