Curve, helix (6)
I believe the answer is:
spiral
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">curve</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that spiral is a more specific form of the action curve)</span><br/ ><br/ >'<span class="explanation-format__original">helix</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that helix can be written as spiral)</span><br/ ><br/ ></span>
(Other definitions for spiral that I've seen before include "Whorled" , "Corkscrew curve" , "Winding like the thread of a screw" , "Mushroom" , "Shape of coil or helix" .)