Dirt spread by insect on doughnut (9)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Anag (spread) of BY INSECT and O. It’s an interesting truth that all doughnuts in crosswordland are O-shaped. Never any jam in them. [Again, as pointed out by FrankieG, the anagrind applies only to BY INSECT, which is then put “on” the O/doughnut, which has to come first in the solution]