Crossword Genius

X, y, or z (7,8)

Ross

I think it could be either:

Which length description are you looking for?:

4

7,8

unknown quantity

(Other definitions for unknown quantity that I've seen before include "Unpredictable type" , "one of those following VW" , "any one of the last three characters?" , "Dark horse" , "may be last in Derby, literally" .)

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