Can I use royal icing under the fondant to glue it?

Royal icing would not be a good crumb coat, you could use butter cream or better yet ganache as your crumb coat. If you use ganache you'll need to use a simple syrup around it for the fondant to adhere to.

What are you trying to glue it too??? The cake? If so there are a couple tricks...if you are talking about glueing it to the main part of the cake, Crumb coat the cake, then pop it in the freezer for 20 min. Take out and very lightly dab just a little water on the bottom of the cake sides, this will keep the fondant in place and make it easier to cover. If you are talking about adhereing fondant to fondant, I use either a little brush with a little bit of water or clear vanilla or almond. I suppose you could use royal icing to add decorations...just a little though. Hope that helps.

Sarah

I decided to make buttercream like zdebssweetsj says.

Sarah, thank you for your tips.

Thank you zdebssweetsj and thank you sarah...

If your new to fondant there are a lot of tutorials here that can save you a lot of stress, when you get a chance to use ganashe you'll love it , it spreads really smooth and sets up very firm which gives you a great surface for fondant, oh yeah it taste yummy too. Good Luck