Line of symmetry (4)
I believe the answer is:
axis
'line of symmetry' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for axis that I've seen before include "Body's line of rotation" , "Imaginary line about which a body rotates and a Dublin theatre" , "Center around which something rotates" , "Turning point" , "On which a body spins" .)