Backhand clears are a difficult shot to master; but if you keep working on the technique, you can get there.
Get your drop shot right first
Your backhand drop shot is the technical foundation for your clear. You need good preparation, and a complete stroke where the arm is rotating.
You should be able to hit your drops deeper into court, well past the service line, even when you’re under pressure. If you can hit like this, then you are ready to develop a technically sound backhand clear. Otherwise, work on your drop shot technique first.
Adjust the contact point
Unlike a drop, the clear needs to travel upwards instead of flat. Change your contact point so the racket is pointing slightly upwards. It should feel like you are throwing the racket out to the side, but also upwards.
This does not mean reaching up for a high contact point (directly above your shoulder). You are still taking the shuttle out to the side of you (as with a drop shot).
Increase the racket head speed
Speed up the hitting action to generate power (but stay relaxed). The rhythm should be
slow, fast. Tighten your grip at the impact point, to transfer power from your arm into the shuttle.
If I could give you just one tip for backhand clears, it would be this: stay relaxed! The more you try to force the power, the worse your technique will get. This is often true in badminton, but especially for backhand clears.
Try not to worry about how far your clears are going, as this will tempt you to force the power. Keep working on your technique and timing, and eventually your clears will reach the back.