I have mastered my slap-shot thanks to my question about lift but would like to ask if there is any good drills that improve my clearing skills
In order to clear the puck effectively, you must pass effectively.
This video shows several drills that include passing with one and both hands with a focus toward positioning on the puck and the follow-through. These drills improve wrist and arm strength, which is vital to effective passing.
As you attempt the following drills, focus on the follow-through. Since you have "mastered your slap-shot," you would know that a lower follow-through will produce a lower puck flight.
Familiarize yourself with banging the puck off the boards and glass. You're never going to clear it up the middle, and an elevated clearance off the boards is going to at least force a blueliner to take time to trap the puck if he can reach it.
If you don't have a partner at the other end of the rink, just take a bucket of pucks and practice quick wristers and snaps at shallow angles off the boards. If you do have a partner, do the same thing but position him by the boards at the blueline and have him try to quickly fire your attempted clearances back at the net. You need to get a feel for what angles, elevations and bounces make it extremely hard for him to field the puck.
A hard clearance helps, but it's more important to make sure the puck is elevated if possible so that it doesn't get easily trapped by a skate or stick. Often a quick flick is simply more effective than a heavy clearance. Your job is to get to the pcuk and then use the boards or glass to bounce the puck to safety.