Is it possible to clean under individual key?

Yes, you can take off the key cap and its possible the wire actuator has slipped (causing it to not spring back).

If you spilled something then I would recommend using a cotton swab (Q-Tip) slightly moistened with distilled water to clean the area. Make sure you don't over wet the swab as you don't want the water to drip in. You can also give the area a second wipe with a good grade of Isopropyl alcohol to clean any oily deposits and to speedup the drying.

Here's a good vid: How to remove and clean Macbook Pro keyboard keys