Well done Bharat. I think the clue is slightly wrong. The usual nominative form is “sedes vacans”, meaning empty seat. Sedes is the singular and plural form. The phrase “sede vacante” is a construction called an “ablative absolute”, setting the circumstance for the clause, meaning “while the see is vacant”.
So “sede” is the ablative declension of “sedes”, and should correctly be followed by “vacante”, the ablative present participle of vacate, the verb for to be empty.
So the clue is technically incorrect, as it’s a hybrid of a nominative and an ablative form. It should either be “sedes vacans” or “sede vacante”.
But the wordplay clearly points to cave reversed in sedans, so I think you have to put “sede vacans” as the answer, even though I would mark it as wrong in Latin!