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.
someone who gets really pissed about a lil name calling and is secretly gay (not that there is anything wrong with that) so tries to hide inside someone elses personality, and plus he is retarded
Wow john, dont be a clonetard, just come out of that closet
A clontarf is a feminized liberal male douchebag; a know-it-all phloom who has to have the last word in every conversation; a guy who uses puns, big words, and trivia questions to show everyone how smart he is; a dick weed who always corrects your spelling. Like a black hole, he sucks all other conversation out of the room so that he can only hear himself talk.