You can fix it the same way you'd fix any ssh problem. Remove Git from the equation and just try to log in using ssh. If it works you should see something like this.
$ ssh [email protected]
PTY allocation request failed on channel 0
Hi schwern! You've successfully authenticated, but GitHub does not provide shell access.
Connection to github.com closed.
If it doesn't work, you need to debug your ssh session. Usually you do this by using ssh -v (and you can add more -v's for more verbosity).
$ ssh -v [email protected]
OpenSSH_7.1p2, OpenSSL 1.0.2e 3 Dec 2015
debug1: Reading configuration data /opt/local/etc/ssh/ssh_config
debug1: Connecting to example.com [93.184.216.34] port 22.
If it hangs at this point, the either you can't connect to the host at all, or it doesn't have an ssh server listening on port 22, or the port is blocked by a firewall.
Since this is Github which definitely has an ssh server and is working for me, I suspect the last one: you're behind a firewall. You can test this by trying to ssh to somewhere else.
In that case either use a VPN to get around the firewall, or try using an HTTPS connection instead.
git clone https://github.com/myuser/myrepo.git