Base watertight might you say? (6)

I believe the answer is:

unholy

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

I can't explain the rest of the clue.

(Other definitions for unholy that I've seen before include "Impious" , "Not sacred; outrageous" , "Godless" , "Irreverent or evil" , "Hellish" .)

I've seen this clue in The Sun.

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