For compass and straightedge problems, are you allowed to use the compass as a ruler? For compass and straightedge problems, you could have a line between two points A and B, and want to make a line the same size between C and line DE.
If you placed the two points of the compass between A and B, and made a circle around C with the same radius, that would achieve this result.
But is this something you are allowed to do?

Yes. Not by the rules about how to use compass and straightedge but because it can be proved that it's as if we could do it (that's proposition 2 of book I of Euclid's Elements).
