Crossword Genius

Perhaps x, y or z (7,8)

Ross

I believe the answer is:

unknown quantity

(Other definitions for unknown quantity that I've seen before include "Dark horse" , "may be last in Derby, literally" , "Candidate not yet examined" , "Unpredictable type" , "any one of the last three characters?" .)

I've seen this clue in the Sydney Morning Herald.
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