Navigational instrument fails to display miles ultimately remaining (6)

I believe the answer is:

extant

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

I don't understand the remainder of the clue.

(Other definitions for extant that I've seen before include "Next at being still existing" , "Still existing, actual" , "left" , "Still in existence, surviving" , "Still going strong" .)

I've seen this clue in The Telegraph.

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