Pretty sure a Tervigon would fall under the role of "combatant." It's in the codex, after all.
Also, you just missed the Tervigon contest: viewtopic.php?f=40&t=13305
A non-combatant would be something like a giant ripper made for consuming biomass, or a tunnelling creature (not a Mawloc), or some kind of walking reclamation pool, or ripper-like organisms that are used to weave sticky webs that trap larger prey, or maybe a non-combatant could be some kind of Tyranid structure.
Long story short, I view a non-combatant as a creature not currently in the codex that plays more of a supportive role in the Tyranid infestation. It's a pretty broad theme that could afford some really interesting entries. Most interesting of all would be Hive Node's suggestion that you write a brief blurb about your non-combatant and the role it plays in the swarm.
I think we need to make a decision on whether or not members can submit models completed before the contest was announced. Yale says nay, Hivenode says yay. Personally, I'd vote for allowing already completed models to enter, but that is in the interest of getting more entries/not forcing members to duplicate creatures like the Swarm Tyrant and the Doom. That being said, even members who have already made these creatures may take the contest as inspiration to make newer, better versions.
I'm also a fan of these more specific contests, rather than "let's make a warrior!", particularly when it comes to things like the Tervigon, Tyrannofex, Swarmlord, Parasite, Doom, etc. These are models that we don't have defined concepts for, which allows artist the greatest range of creativity. What's more, it means we have that many more interpretations of the creatures to draw on when we eventually decided to make our own.
And I'm throwing my hat in with Tyranid Primes as the next contest subject.