X, Y or Z (4)
'x y or z' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for axis that I've seen before include "where wheel turns?" , "line to be turned round" , "Line about which to rotate" , "Centre about which something turns" , "Imaginary line around which a body rotates" .)
(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" .)