Highest charge (5)

I believe the answer is:

prime

I believe this is a double definition.

'highest' is the first definition.
(thesaurus)

'charge' is the second definition.
(thesaurus)

(Other definitions for prime that I've seen before include "put in the picture" , "Of superior grade" , "Two or three, for example" , "Prepare for use, say a pump" , "Monastic service; best quality" .)

I've seen this clue in The Telegraph.

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