Can I use an iMac as an external display for my MacBook Pro in clamshell mode?

I have a Late 2009 iMac that I'd like to use as an external display for my Mid 2014 Retina MacBook Pro. I'm familiar with using clamshell mode with the MBP on an external monitor, but I'm curious if the same feature can be applied on the iMac. I've seen target display mode in YouTube videos (using the iMac as a second screen), but can't find any videos or articles explaining whether or not this would work if the MacBook was in clamshell mode.

Thanks!