No, but you could certainly hit with an attack multiple times during your first turn with the Extra Attack feature or similar. The second clause ensures that the extra damage only applies to the first (successful) attack.

It is not possible to have multiple "first turns in combat" in a single combat, but you could certainly hit with an attack multiple times during your first turn with the Extra Attack feature or similar. The "once per combat" clause ensures that the extra damage is only applied to the first successful attack.

No, but you could certainly hit with an attack multiple times during your first turn with the Extra Attack feature or similar. The second clause ensures that the extra damage only applies to the first attack.

No, but you could certainly hit with an attack multiple times during your first turn with the Extra Attack feature or similar. The second clause ensures that the extra damage only applies to the first (successful) attack.