There are two key factors here. The first is having the correct ratio of butter to flour in the topping. The second is how you work the mixture together. Start working with the butter and flour alone until you get the consistency you want before adding in the sugar, lemon zest and other seasonings. If I add nuts I always add them on the top rather than mixing them into the crumble topping as they can interfere with the consistency. You should work the mixture until you get a crumbly but chunky texture. I form the mixture into balls and let them break away naturally into crumbly chunks. I also continue this technique when I assemble the crumble to ensure any small crumbs are pushed into chunks.