What the Earth spins on (4)
I believe the answer is:
axis
'what the earth spins on' 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 "Line from N. to S. Pole" , "wartime alliance" , "x or y line on a graph" , "Imaginary dividing line" , "line to be turned round" .)