This meaning of the word is used exclusively in the AFOL, or Adult Fans Of Lego community. It's a combination of the words clone (as in Star Wars Clone Troopers) and (re)tard.
The term refers to someone who buys a lot of the standard clone trooper figures, and then uses (or rather: abuses) his sharpies while trying (and most of the time failing) to draw cool legion colors on them. They most often adopt online names like "Commander Marks of the ### (fill in any 3 numbers) Legion" and build very shoddy and ugly builds. They react very defensively when somebody calls them a clonetard.
"Commander Jigsaw of the 249th Legion"? Geez, not another one of those clonetards!