It is a heavily modified Clark 15B made to fire "ESP" (Explosive Sticky Projectile) timed explosive RDX rounds. When they explode they damage anything in their radius and can even damage vehicles, albeit only slightly.
Overall effect is similar to the PK-74 AR-Rocket, but the sticky projectiles are harder to use because they travel in a parabolic arc and have a timed explosion, which means the enemy can simply run away from them.
The Clark 12-RDX also allows Support soldiers to dispose of enemy explosives. Two rounds are sufficient to clear any mines or RDX Demopaks.
Despite being a weapon in-game, the 12-RDX is considered a gadget and as such occupy the equipment slot, allowing the player to still carry either a machine gun or the Clark 15B shotgun as a main weapon.