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