Centre of rotation (4)
I believe the answer is:
axis
'centre of rotation' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for axis that I've seen before include "Straight line" , "Imaginary line through the centre of an object on which it spins" , "line to be turned round" , "Line from N. to S. Pole" , "Fixed line in revolution" .)