the third prong (ground prong) on my amp's power cord broke off, is it safe to use it with the missing prong?
my guess is thats the earth, so no, not safe - if it works at all.
DON'T!!! get it taken to an electrician/tech ASAP and fix it! or suffer a painful death...your choice
You can get the replacement piece for $3-5 and replace it easily. Then you have a more solid plug than before too. There is no charge in it when it's unplugged anyway, I actually had this same situation and just clipped the end off and installed the new one in 2 minutes.