He's not a newcomer, but could Lacy Clay get his district cracked enough for a GOP win in 2022?
No.
Clay's district is extremely Safe Democratic; it is based in heavily black St. Louis after all, which has not voted Republican since the 1920s. If anything, Emanuel Cleaver (though also not a newcomer) may be in jeopardy, as Republicans could try to crack Jackson County, and divide it between the Safe Republican districts adjacent to Kansas City. I don't think they will, however, as such a move could backfire on them.