+ You may play an Action card from your hand that you don't have a copy of in play. If you do, +1 Action.
Conclave is an Action card from Nocturne. It's a conditional village that only gives extra Actions if you can play an Action card you don't yet have in play.
When you play this, you can play an Action card from your hand, provided that you do not have a copy of that card in play.
It does not matter if you played the Action card this turn, only that it is not in play when you play Conclave; you can use Conclave to play a card that you played but trashed and so do not have in play, like a Pixie you trashed, but cannot use it to play a card you did not play this turn that is still in play, such as a Secret Cave from your previous turn.
Conclave normally cannot play a Conclave, as that is a card you have in play.
If you do play a card with Conclave, then Conclave gives you +1 Action, which has no special limitations, and so can for example be used to play another Conclave.
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+. You may play an Action card from your hand that you don't have a copy of in play. If you do, +1 Action.