This is Fun level 5, "My first teleporter". You have 20 lemmings, but you have to finish the level with 21 of the, so of course there must be a cloning machine. You only have 2 minutes to finish this one.
Let's go! Start by increasing the rate the lemmings come out to about 60 or so.
As the lemmings come out of the entrance, give each one an umbrella.
Make the second-to-last lemming into a Digger. The rest of the lemmings will walk into a teleporter and end up at the top platform. They will collect a Basher for you.
He will dig down and become cloned. The rest of the lemmings will begin exiting the level.
Make the cloned lemming a Basher to dig rightward.
He will then join the other lemmings and exit the level.