Take the "necessary and proper" clause again, because it's not only a vital example but a good one. Here's what it says:
At the time the Constitution was ratified, Anti-Federalists were up in arms that the clause could be interpreted to give the federal government basically limitless power, while the Federalists argued that the clause only granted the authority to carry out explicitly granted powers, and that without it the Constitution would be toothless.The Congress shall have Power - To make all Laws which shall be necessary and proper for carrying into Execution the foregoing Powers, and all other Powers vested by this Constitution in the Government of the United States, or in any Department or Officer thereof.
The Federalists won the argument and the clause stayed, but the debate over it's meaning continues today, with liberals (the Federalists, in other words) arguing for a broad reading of it and conservatives favoring the 10th amendment.
So don't sit there and say the Constitution "defines terms clearly". That's malarkey. In some places it's very clear but in some very important spots it's incredibly vague. A huge portion of the authority the federal government wields right now can be traced to "necessary and proper" and also the Commerce Clause (which gives the government the right to "regulate" commerce between the states). But what does "regulate" mean? Or "necessary"? Or "proper"?