Curving in and out (7)
I believe the answer is:
'curving in and out' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sinuous that I've seen before include "Wavy -- winding" , "Supply bending; wavy" , "Having many curves and turns" , "Full of curves, bends" , "Twisting and turning" .)