Current navigator's device needs to start (6)

I believe the answer is:

extant

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'current' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for extant that I've seen before include "Still living" , "Still existing, not obsolete" , "Not destroyed or lost" , "still alive" , "Net tax still existing, not gone" .)

I've seen this clue in The Telegraph.

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