You can't use a locking clip with a lap only belt, but it should lock on it's own. I'm not sure how big the back seat is, but the true fit may be hard to get right next to another seat. But if it fits, the true fit's lockoffs should take away any worry with the lap belt anyway. But in general, lap belts are totally fine to install with, and some find they are easier to work with even.
Mom to a girl - 6, a boy - 4, and Baby - 2