Line of rotation (4)
I believe the answer is:
axis
'line of rotation' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for axis that I've seen before include "Imaginary line about which a body rotates and a Dublin theatre" , "Centre around which something rotates" , "Turning point" , "Imaginary line through the centre of an object on which it spins" , "Imaginary line around which object rotates" .)