Right, so "Suppose" is the definition — that's PRESUME, meaning to assume something. Now for the build: "Trump" gives you PRES, short for PRESident, and "Big Bird" is a cheeky reference to an EMU, which "turning up" (reversed, since this is a down clue) becomes UME. Stick UME underneath PRES, since it's supporting it, and you get PRES sitting on UME — PRESUME.