A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 15 January 2014
In the new wording for Come One at a Time,
While there is exactly one defending Personality, he has the ability, “Repeatable Battle: Challenge a target enemy Personality. Destroy the loser” and each player has the ability, “Repeatable Battle: Move home your target attacking Personality. Bow him as he moves.” Battle: Destroy a Terrain (if able). Put this Terrain into play. Take an additional action from it
the "while there is only one defending personality" is a clause that unlocks the printed effects to follow, correct?
That said, if either player plays this card while the defender has more than one personality at the battle, does the additional action fail because it must be one of the actions provided by Come One at a Time?
Jon Palmer (rules official), 15 January 2014
In the new wording for Come One at a Time,
the "while there is only one defending personality" is a clause that unlocks the printed effects to follow, correct?
That said, if either player plays this card while the defender has more than one personality at the battle, does the additional action fail because it must be one of the actions provided by Come One at a Time?
Correct on both accounts. If the Defender has more than one unit at the battle, he may not take the abilities granted by Come One at a Time, but he can still use it to destroy a terrain, failing to get the additional action.