The Nuclear Cluster Missile is a missile added by the ICBM mod. The "Cluster" portion of the missile comes in to play as it approaches its final destination. You can observe the missile breaking into 4 smaller missiles just before impact. It acts similar to the fragmentation explosive because as it detonates it fires shrapnel in all directions exploding with anything that they (the shrapnel) comes into contact with. The Nuclear Cluster Missile is a Tier 3 (T3) missile and may only be lauched with a Launcher Platform. As with normal nuclear explosives they leave behind toxic radioactive fallout that spreads from living block to living block. Radiation poisoning will affect any nearby players/NPCs at random durations.
It is not advisable to spam-fire these on servers or singleplayer, as it can cause tremendous lag.