To my knowledge there is no single attribute in Spectrum that contains the aggregate value. It is computed internally.
For a watch, you can compute it in the expression. For example, if you know there are 4 CPU's on the device and they are instanced 1, 2, 3, 4, your expression could be the following:
((NRM_DeviceCPUUtilization.1 + NRM_DeviceCPUUtilization.2 + NRM_DeviceCPUUtilization.3 + NRM_DeviceCPUUtilization.4) / 4 )
There are issues with this though:
1. Not all devices have 4 CPU's. Some have fewer and some have more
2. The instances may not all be 1, 2, 3, 4.....
Another option is to compute it externally with a Command Line Interface (CLI) script and have the script generate an event.
Joe