Curve, helix (6)
I believe the answer is:
spiral
This clue is a double definition.
'curve' is the first definition.
(I know that spiral is a more specific form of the action curve)
'helix' is the second definition.
(I know that helix can be written as spiral)
(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" .)