Crossword Genius

Underground cell like this crossword, mostly (5)

Ross

I believe the answer is:

crypt

'underground cell' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for crypt that I've seen before include "Burial vault" , "Catacomb" , "Underground room of church" , "Church's basement" , "Underground burial chamber" .)

I've seen this clue in The Guardian.
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