The 2016 equivalent of the mid-late 2000s phrase "you got served". For one to be put on clams, they must be showed up by somebody who is considered of lesser ability (i.e. an underdog).
The phrase's origin is unclear but it did originate from the suburban town of Montville, NJ. The thought process behind the phrase seems to tie into the nature of clams to be served as an appetizer (i.e. clams casino) on a platter for group consumption. In this example, it points to somebody being put out in the open to be picked on by a multitude of bystanders.
Mark, the tiniest kid in the school, just stuffed Rob's (the varsitybasketball captain) shot in gym class. ROB JUST GOT PUT ON CLAMS
" (the game is pretty difficult). in fact, its just a smidge away from being too hard--newbies can expect a full-on, metroid-clamped-on-your-balls evisceration--but vets will undoubtedly enjoy the rich, meaty challenge."