The ad block comes first, but you want it to look like it's inside the .entry box?
If you know the height of the ad box, you could give .entry enough top padding to push down the main content inside, making visual room for the ad block.
Then the add block could be taken out of the flow with positioning... here, since .entry isn't a parent, you'd probably want to just use the known height of the ad box to decide how many px (or ems) lower you want it. So possibly relative positioning, with top: value set.
You wouldn't want or need to float it though.
I suppose if I had a better idea of the setup, I could come up with a better solution. This one could be ok, but it's brittle.