Can a creature blocked by multiple creatures have its combat damage assigned to only one of the blockers?

When a single attacker is blocked by multiple creatures, the attacking player must assign combat damage to those blockers in a specific order. First, the attacking player declares a damage assignment order for the blockers (CR 509.2). Then, when assigning damage, the attacker must assign at least lethal damage to each blocker earlier in that order before assigning any damage to the next blocker in line (CR 510.1c).

"Lethal damage" means enough damage to kill the blocker — specifically, damage greater than or equal to the blocker's toughness minus any damage already marked on it (CR 510.1c, 704.5g). Only after satisfying lethal damage for the first blocker in order can the attacker assign damage to the second, and so on.

So the attacker cannot simply choose to put all damage on just one blocker while leaving others unharmed, unless the earlier blockers in the order have already received lethal damage from other sources, or the attacker has enough power to satisfy lethal damage for all preceding blockers first.

Example: Your 5/5 creature is blocked by a 2/2 and a 3/3. You order the 2/2 first. You must assign at least 2 damage to the 2/2 before assigning any to the 3/3. You could assign 2 to the 2/2 and 3 to the 3/3, or 5 to the 2/2 and 0 to the 3/3 — but you cannot assign 0 to the 2/2 and 5 to the 3/3.

Note: Abilities like trample or specific card effects can modify these rules, but without such exceptions, CR 510.1c strictly requires lethal damage assignment in order.