Skip to the 7:02 mark here for a video demonstration.
Single-leg Romanian deadlifts are awesome for increasing length of the hamstring and adductors on the stance leg.
They are also great for increase in functionality of the stance leg hip.
Improved functionality will directly improve the mobility of the hip.
Before you jump straight to weights, make sure you can do it with complete balance without weights.
Balance is the top priority.
Don't add weight if you can't perform complete motion without weight.
Drive "long" with the leg and the torso will follow.