Clear as in see-through? There's not really anything called "clear primer". There is adhesion promoter in automotive paints.
Otherwise, primer is primer... Krylon, Rustoleum, Plastikote... They all have primers of varying differences. Some are self-etching, some are just regular old primer, and some are a little thicker.
There are then plastic primers from companies like Tamiya, etc. For specifically plastic models and things. They're not cheap though comparatively speaking, but they're formulated to be fairly thin as not to obscure details on model kits. I've found automotive primers often work fine though too.
