It is not a toenail it is a claw.such as- a dew claw that they have on the back of their leg.if the dog isn't in pain ,then have someone help you hold(make sure to muzzle the dog so it can not snip at you/bite you if it does feel any pain) the dog and clip the claw the rest of the way off and treat with betadine,or medicated salve.if you are not able to do this yourself take the dog to a vet ,they may even do it for free if you call ahead and check with them.at least this is what I would do.maybe even reverse that and call the vet first and ask them how to cut the claw off correctly so you don't harm the dog.