Plain as a pikestaff means it's obvious; very clear.
Also, If something is as plain as a pikestaff, it is very clear.
Mahesh: Why does the Senator always vote for measures that would help the tobacco industry? JANE: It's as plain as a pikestaff—because there are a lot of tobacco
growers in his district, and he wants them to vote for him.
Kiran: I have a suspicion that Marcia is upset with me.
Rahul: A suspicion? Come on, Fred, that's been plain as a pikestaff for quite some