You might find you have two separate problems here. The plastic clip is one thing, but there’s no way a passenger seat would be locked into place with a plastic component. It will almost certainly involve a metal ring or clasp mounted to the floor around which the locking mechanism on the seat locks.

Here’s a possibility, though: perhaps the broken piece of plastic from the broken clip has somehow managed to work its way into the seat locking mechanism and is not allowing the seat to fully clamp into position. Fold the seat up and have a good look around at the clamps or locking rings in the floor. Even if it’s not the errant piece of plastic, you might find a build up of dust or mud is not allowing the seat to find its locked position.

The other chance is that the locking mechanism has somehow closed onto itself before it has found the locking ring. If that’s the case, you may be able to take a screwdriver and pry the lock open before trying to lock the seat back into place. Be careful, however, as these locking mechanisms are often spring loaded and will crush a finger placed in the wrong spot.