Try keyframes at start, one-third, two-thirds and full rotation. That'll be 0, 120, 240 then 0 degrees again.
Thanks for replying conroy,
Well, PS only lets you rotate up to 180 or -180 degrees for starters, and also if I do like you suggest with 0* at the start and end, it will just cause the last rotation to turn counterclockwise back to the initial position of the smart object, which is not what I want. I want the smart object to keep rotating clockwise. But thanks anyway for the input, and maybe I've misunderstood in some way.
In the meantime, I have actually figured out a crude way to achieve the continuous clockwise rotation of the smart object... Simply use the Shift key and rotate in 15* increments, making keyframes along the way until the layer is rotated back to it's initial position. Unless someone else has a better method, it looks like I'll have to live with that.
You can rotate by up to +/-180 deg at a time, so what was to stop you doing 120 deg at a time, similar to my suggestion, instead of 15 deg at a time?