underdoer

underdoer

(ˌʌndəˈduːə)
n
a person who underperforms or fails to do what is necessary
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014