Mysterious underground vault I see (7)

I believe the answer is:

cryptic

(Other definitions for cryptic that I've seen before include "Obscure or puzzling (as our page neighbour Crosaire)" , "Obscure or puzzling (as our neighbour Crossheir)" , "Enigmatic" , "Esoteric" , "(Of crossword) withindirect clues" .)

I've seen this clue in The Sun.

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