Either way you're breaking down the tunica. If you clamp first, the tunica will become very pliable and soft, making it easier to stretch. On the other hand, doing length first will also fatigue the tunica making it easier for your girth to expand. As an example of this, one day I decided to do 45 minutes of A-stretches followed by clamping, and the first 5 minutes of clamping was the most soreness I ever felt from Penis Enlargement, so there's definitely fatigue there.
That being said, I think it would be best to stretch first, as it is kind of tough to comfortably grip hard for stretching after an intense girth session.