Crossword Genius

Prime Suspect claimed ID left unused (4)

Ross

I believe the answer is:

acme

'prime' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for acme that I've seen before include "Zenith - optimum" , "crown" , "Point of maximum development" , "Highest point of excellence or achievement" , "'Highest point, apex (4)'" .)

I've seen this clue in The Guardian.
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