It's normal for new cables to stretch a bit in use. You have two places to adjust it: At the handlebar (fine adjustment) and at the clutch housing (larger adjustment). Usually, you'll want to work at the handlebar.
1. Slide the rubber moisture cover back. You'll see two knurled hardware bits, large and small.
2. Start by loosening the larger bit (it's the locknut). Once that's loose, turn the inner hardware bit counterclockwise to take most of the slack out of the cable (leave a tiny bit though - don't know the spec offhand.)
3. Then, while holding the inner/smaller hardware bit, tighten the outer, larger one. Tighten it firmly so it won't loosen by itself, but not too firmly, so you could make another adjustment if needed.
4. Slide the rubber boot back on.
If you need to lubricate the cable, you'll want to go through this drill to loosen everything up, to allow lubricant to be injected/sprayed in. There is a neat tool for this that slips over the cable sheath end, and has a hole for a spray can tube. Your local bike shop will have these.