Line of rotation (4)
I believe the answer is:
axis
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">line of rotation</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)