Am I able to store my files in iCloud only (no local copy)?

The general answer is YES, but it's not a good idea. iCloud is really not meant as a backup solution. It is meant as a syncing solution between your various computers and iDevices.

To get any more specific, you have to tell us exactly what files are you talking about: music? photos? documents? backups?

I've got a 200GB iCloud Drive account. I'd like to store files (a bunch of documents and development files) on my iCloud Drive without having a local copy to save space on my MacBook Pro. Is this possible?

You can use the web interface at iCloud,com to upload and download to iCloud drive, but then you will see your files only using the web interface https://www.icloud.com and not in the Finder on your Mac:

The main purpose of iCloud drive is to have a central storage, that will be used to keep the same files updated on all your devices, that are using iCloud Drive.Kt is essentially an external drive, mirrored on he Macs that have iCloud Drive enabled.

All devices will keep local shadow copies of all files and folders on iCloud Drive, and when the documents change in iCloud the local copies will be updated. The local shadow copies will be stored in a hidden folder in your system library, and you can access them using the iCloud icon in the Finder sidebar, even if you are offline and not connected to iCloud.

See: iCloud Drive FAQ

iCloud Drive FAQ

With iCloud Drive, you can safely store all your presentations, spreadsheets, PDFs, images, and any other kind of document in iCloud. Documents you store in iCloud Drive will be kept up to date across all of your devices, and you can access them from your iPhone, iPad, iPod touch, Mac, or PC.

Here's what you can do with iCloud Drive:

• Store and access all of your documents in one place from any of your devices
• Keep files and folders up to date across all your devices
• Create new files and folders from iCloud-enabled apps
• Work on the same file across multiple apps

You could disable iCloud Drive on your Mac and only access it using the web interface at https://www.icloud.com, but that would be very cumbersome, because the access will be slow and it will be tedious to make backup copies of your cloud documents, if you can only use the web interface. If you just want to save space on your MacBook use a small portable drive. That will be quicker than going through the web interface.