(assuming after effects) If you can't get an alpha pass of it from the render as gubar mentioned, best bet would be to throw a simple combination of a feathered elliptical mask (animated to fill most of the fire) and a luma key (removing darker, tweaked to taste). The edges of the flame would would be unaffected by the add mask, and so would fall off naturally according to the luma, but the center (presumably with black smoke as in the above) would be entirely visible due to your keyframed mask. It may sound complicated but it's really quite simple to set up.