Make a fist with your left wrist. That's what a wrist cock is. Your best bet is to just let it happen naturally, or, make swings very slowly, focusing on that aspect.
There's nothing set in stone about HAVING to have your shaft parallel to the ground at the top, or having a completely straight left arm. Look at JB Holmes- he only has a three-quarter swing, with a "soft" left elbow, but he's one of the longest hitters on Tour!
It's really about body turn. Every pro has a different angle in their wrists... but it's the amount of body turn that really gets the shaft in that position. Watch a video or swing sequence pictures. On average, when a pro gets his hands at about 9:00, their wrists are already hinged. Their body is also turned about 45* from where they started. When they get to the top, their hands and arms are still in the same spot, more or less, relative to the chest. But look at their body turn- the hips are now at 45*, and the chest is 90*. Fred Couples is a good model to use to get a visual of what I'm talking about. As the pro comes down, that hinge they had going up is still there going down. They release it milliseconds before impact (which also helps make a divot on the target side of the ball).
As far as the elbow is concerned, that's more a product of your physique. If you're a little hefty or barrel-chested, it'll be harder to keep that elbow straight at the top. It's naturally going to bend. That's OK- chances are, it'll also naturally straighten itself out as the downswing starts.
I believe the key is to feel the club lag as it's heading towards the ball. Try gripping you're club like normal, but then take your right index finger and thumb off. Make a normal swing. You should be able to feel that lag, like a kink in your wrist. If that doesn't work, you could also try making a slow swing and stopping when you get to the top. Let go of the grip with your right hand and hook your index finger around the shaft. Slowly start the downswing. With your finger hooked on the shaft, you should feel (and see) what lag looks like. As you get near impact, let go with your finger. You should also now be feeling that whipping action through impact- a key to increasing clubhead speed and distance.
