If your tendency is to bend your left arm at the top of the swing, it may be because of lack of flexibility in your shoulder and chest muscles as well as an inablitly to rotate your torso. Your arm has to bend to place the club at the top of the backswing.
Hold the club with just your lead arm.
Place your other hand on the lead arm wrist.
Swing the club to the top and physically pull your arm straight as you take the club to the top.
Hold the position as you feel a stretch in your shoulder and chest.
Repeat for 8 to 10 repetitions.
Take care to check your grip to make sure that it is in the fingers of the lead hand and not the palm. When the grip lays in the lead hand palm, it restricts your ability to hinge your wrists, so you end up hinging your elbows instead.
This drill can be done with or without hitting a ball. This is an isometric exercise that will not only train the proper movement but will tone muscles in the shoulders and chest.