Before I say anything else, I have to say that almost universally the best aspect to improve for singles is footwork. This is true at all levels of play. The player with the cleaner and faster footwork will have a huge advantage.
In my experience, singles is far less forgiving of mistakes. In doubles you can take a lot of risk and push the pace with less concern of losing control. If you are ever off balance, you still have a partner to cover you. If you play a short lift under pressure, at least you’re defending together. In singles, every time you are off balance, or every time you hit a sub-par shot, you’re likely to immediate lose, if the opponent knows what they’re doing.
So, it is more important to stay on balance and reduce mistakes in singles than in doubles. That is not to say you shouldn’t try to create problems for your opponent, just that it shouldn’t be at the cost of your own composure.
The length of your clears and lifts is definitely more important in singles compared to in doubles, but I wouldn’t necessarily call it defense. In doubles, attacking lifts and clears are far less common, but in singles lifts and clears are definitely not defensive by default. I find in my own games, I prefer to play shots to the net when I’m in trouble, specifically because it feels safer. The purpose of those shots is defensive, to get me out of trouble. Similarly, nearly all smashes I will defend to the net. Conversely, many lifts and clears are attacking the deep corners, trying to make the opponent hit shots from behind them, especially on the backhand.
Practicing the consistency of shots to the back lines will definitely improve your singles significantly, just because failure to hit quality shots is punished more harshly in singles. In a similar vein, accuracy of shots is more important. In doubles, hitting close to the sidelines is often not worth the risk, because 2 people are defending the court, so usually smashing directly at the players or especially in between is a better risk-reward option. But in singles, where the opponent needs to cover the entire court by themselves, hitting smashes and drops close to the sidelines can be very rewarding, so practicing your accuracy and consistency on those shots is a good idea if you’re specifically training for singles.
But again, improving footwork technique and speed will almost certainly be the most effective way to improve your singles game.