Assuming it's a normal git clone, I'm afraid that they're not resumable, as far as I know. To add support for resumable git clone / git fetch has been a suggested project for the Google Summer of Code in the past.
One exception is if you're actually doing a git svn clone, then you can restart it by changing into the directory and running git svn fetch, but I assume that this is just a normal git repository you're cloning. For some other ideas of how to work around this, you might want to try the suggestions in the answers to this question:
- How to complete a git clone for a big project on an unstable connection?
