Type of staircase (6)
I believe the answer is:
spiral
'type of staircase' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for spiral that I've seen before include "Whorled" , "Snail shell form" , "Ever widening curve" , "Sir, pal winds in continuous loops" , "Shape of a coiled spring" .)