If you can only make one Attack in a round, can you still decide to use a full round action to make the attack gaining the benefits of say the Shield of Swings feat or the Defensive Parry ability of the Aldori Swordlord fighter archetype?
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You need only to have and use a full round action, in terms of action economy, in order to take a full round action.
Having more than one attack because of BAB of +6 or higher, or TWF, or haste, or any other reason, is not a pre-requisite to be able to take the full round action in general or a full attack action in particular.
But if the reason you can only take one attack this round is because you have already used a move action and only have a standard action left, then you can't take a full round action simply because you don't have a full round action to spend. You can't move 30-feet and then use Shield of Swings simply because you don't have a full round action to spend.