Conductor Clearance Measurement tool is located in the left hand tool bar.
Use this tool to measure conductor clearance to anything other than ground, for example:
When in conductor clearance measurement mode, all enabled environments will be shown. To ensure clarity when taking measurements, go to Environments and check only the required environments are enabled prior going into the conductor clearance measurement mode.
Left click on any conductor to select it, then hover to another object to see clearances between the two. Right click to de-select.