Take a look at this...
http://www.geocities.com/rich_is_bored/m_cloud.zip
(Right Click - Save As)
It's just a particle emitter and two wind FX. Make a preview to see it in motion.
The first wind FX is a linear draw. The second wind FX is a doughnut shape.
Of course realistically the ploom of the cloud is going to slowly rise so you'd need to animate the position and scale of the two wind FX but it should be a good starting point.