Helical shape (6)
'helical shape' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for spiral that I've seen before include "Helical shape" , "Helical curve" , "Shape of coil or helix" , "Shape of a corkscrew or spring" , "Whorled" .)
(Other definitions for coil that I've seen before include "Eg, wire wound round and round" , "Curl, loop" , "Twist" , "Curl, spiral" , "Loop - conductor" .)