Plot line? (4)
I believe the answer is:
axis
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">plot line?</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for axis that I've seen before include "Alliance of powers eg Germany, Italy and Japan in WWII" , "WW2 Nazi alliance" , "vertebra" , "Line of symmetry" , "Imaginary centre of rotation" .)