Cloning build definition is a feature that comes with TFS Power Tools.
I haven’t tried it but it seems to be part of the beta version for 2012 available here: http://visualstudiogallery.msdn.microsoft.com/b1ef7eb2-e084-4cb8-9bc7-06c3bad9148f
(updated url to published version. Thanks Bernard Vander Beken)
Visual Studio 2013 version:
http://visualstudiogallery.msdn.microsoft.com/f017b10c-02b4-4d6d-9845-58a06545627f
(Credit to Bernard Vander Beken)
Visual Studio 2015 version:
https://visualstudiogallery.msdn.microsoft.com/898a828a-af00-42c6-bbb2-530dc7b8f2e1
(Credit to Bernard Vander Beken)