The CPU's in modern smartphones don't run at their flat out full core CPU GHZ unless taxed, they will normally step down regardless.
All underclocking does is allow it to step down further or not reach as high.
I would just stick to how the device is meant to be used - and switch off some background stuff such as google now (especially location history) and turn your emails from PUSH checking to automatic 30mins instead. Turn off bluetooth if your not using it, likewise GPS.
Simple things like that will garner better battery life than messing around under clocking the cpu and potentially putting the device in harms way.
Besides you have a one plus one. You should be able to get a decent day of heavy usage out of it - unless your one of the affected users where the new firmware reduced battery life, in which case no amount of under clocking will help, you just need cyanogen & oneplus to fix it.