Combine clam juice and saffron threads in small bowl. Melt butter in heavy medium skillet over medium heat. Add mushrooms, sprinkle with salt and pepper and cook until beginning to soften, about 3 minutes. Add half of green onions and 1 tablespoon tomato paste and stir to blend. Add clam juice mixture and simmer 2 minutes. Add crab and tarragon and stir until heated through. Season with salt and pepper.
Meanwhile, cook linguine in large pot of boiling salted water until just tender but still firm to bite, stirring occasionally. Drain well. Return pasta to same pot. Add sauce and toss to coat. Divide pasta between 2 plates. Sprinkle with remaining green onions and serve. Serves
