One of the requirements to use clamshell mode is to have the power adapter plugged in. Is their any file modification or app that will work around this? Like to switch throughout the day from a sit down desk to stand up and bringing the power adapter back and forth, is becoming more of a hassle.
You can use Insomniax to disable sleep when the display is closed. This will mean that you can close the lid and display the screen on a secondary monitor mode without needing to have a power adapter connected as the Mac will not sleep when you close the lid.
Insomniax is now abandonware and no further updates or binaries will be supplied. Further info.
An extension called nosleep does exactly this. Free and easy to set up!