clean macbook air keyboard

The ideal solution is to purchase a new space bar (plastic key cap). They just snap on and they're inexpensive. Make sure to positively identify your specific model MBA.

Cleaning: refer to How to clean your Apple products - Apple Support.

Having said that isopropyl alcohol (rubbing alcohol) will remove tree sap in general. Just use caution to prevent anything from dripping down from atop the space bar and into the keyboard mechanism. Do whatever you have to do to accomplish that. Removing the space bar key cap would be best. Liquids will ruin the keyboard and such damage cannot be fixed.

Also, don't close the lid until you remove the contaminants, because that's certain to contaminate the screen and you'll have more problems.

Lens cleaner is fine, since it's merely a cloth soaked in the exact same thing: isopropyl alcohol.