If I clear files from Dropbox do I then regain storage space?
Yes, you regain storage space when you delete files from Dropbox.
Also, with Dropbox, if you had lots of pending camera uploads that couldn't be uploaded to your Dropbox because it was full. Once you have cleared some space, those camera uploads will start to be uploaded to your Dropbox again.
Yes, you can regain storage space by deleting files in your Dropbox. Already deleted files don't count and permanently deleting them will not change anything.