Combine the squash, turnip, leek and raisins in a 6-quart slow cooker. Season the chicken with salt and pepper and place on top of the vegetables.
Whisk the chicken broth, tomato paste, cumin, coriander, turmeric, lemon zest and juice, and 1 teaspoon salt in a large bowl. Pour the mixture over the chicken and vegetables. Cover and cook on high, 4 hours.
Serve the chicken and vegetables with couscous; drizzle with some of the cooking liquid. Top with cilantro.