hda6 is full ... how to clean it?

root@host [~]# df -k|sort -n|tail -100
/dev/hda1 101089 23459 72411 25% /boot
/dev/hda2 9920624 1840752 7567804 20% /usr
/dev/hda3 6940516 2192824 4389444 34% /var
/dev/hda5 988088 16424 920660 2% /tmp
/dev/hda6 988088 979348 0 100% /
/dev/hda8 55772668 22399484 30494340 43% /home
Filesystem 1K-blocks Used Available Use% Mounted on
none 223664 0 223664 0% /dev/shm
/tmp 988088 16424 920660 2% /var/tmp