How to melt glucose?
If it's for a pie, by melt the recipe author probably meant to dissolve in the least amount of water.  You can do that by making a glucose syrup, as Pulse mentioned.  Place one cup of dextrose with a third of a cup of water in a pot and heat until dissolved.  Commercial glucose syrups are typically 10 to 25% water.

If you melt solid glucose in a pan, which happens at about 146&deg;C, it will burn.