You are like the turtle posture, opponent is upside your back, he try to take your left collar from under armpit.
Grip his left wrist with your right hand, seize his left arm with armpit.
In this case, you should seize inner part rather elbow.
With seize his arm , you grip his pants, your head stick out from his left armpit, you can bend his joint, like push up his armpit.
You are griping his pants, prevent his rolling escape. You should keep his arm away from his body.