I think most of the store bought kinds that have crunchy clusters do have lots of sugar, which give them the candy texture. The more naturally sweetened ones always seem softer and unclustered. I've tried recipes for granola
bars with beaten egg whites in them, which helped to give a crunchy texture. You might try something like that if you want to keep the sugar level down. I've never been able to make clustery home made granola, but I also like to use very little sweetener. (Agave nectar works very nicely. It is sweeter than sugar, so you use less, and it doesn't mess with your blood sugar levels.)
I also looked online and saw
usaweekend.com recommend adding some water and squeezing the oats together in your hand before baking them in the pan. maybe that, with enough oil/sugar would make crunchy clusters? There isn't too much sweetener in the recipe given.
Also, not sure but maybe quick-cooking oats would be more conducive to clustering and getting crunchy. ...on second look, the website that advises using water says they cluster better, but have a powdery texture and taste like raw starch.
Edit: Oh and one other recipe I saw said to tightly pack the granola right after you take it out of the oven, then break up when it cools, which causes clusters.
This recipe had a lot of sugar, though:
6 cups uncooked oats
1 cup shredded, sweetened coconut
1/4 cup whole wheat flour
1/4 cup wheat germ
1/2 cup powdered milk
2 Tbsp cinnamon
1 tsp powdered ginger
1 tsp cardamom
3/4 tsp. salt
2/3 cup light olive oil
3/4 cup dark corn syrup
1/4 cup honey
1/4 cup packed dark brown sugar
2 Tbsp. vanilla extract
1 Tbsp. coconut flavoring
1 tsp almond extract
1 Tbsp. walnut flavoring
1 Tbsp. maple extract
1 tsp orange extract