Clone a repository partially (sub-folders)

How do I clone a repository hosted in BitBucket partially with Mercurial? Let's say, myrepos has several sub-folders and directory structure within it. I want to clone only one-subfolder, and quickly make a fix in one file and commit it back. Mercurial does not seem to recognize <username>/myrepos/directoryI as a valid repository. If my repository is large, and I quickly want to clone and fix one particular file, is there any option to do that? Does anybody else have a similar problem?