I spilled coffee on my MacBook Air and wondering if Apple will clean it for free.

There may be more involved than just cleaning the Mac's keyboard. If that coffee spill entered into the laptop, especially if you use cream/suger/etc., that spill may have evaporated, but it could leave conductive residue behind which can potentially further damage the laptop.

AFAIK, Apple does not provide "cleaning" service for their devices. They leave that up to us. However, in this case, I highly suggest that you make a Genius Bar appointment at your local Apple Store or Apple Authorized Service Provider to have them take a look and provide you with your options at no cost to you.

Any potential repair costs will depend on, whether or not, you added an AppleCare+ plan when you purchased this laptop.