Winding stair's top pair turned left (6)
I believe the answer is:
spiral
'winding' is the definition.
(I've seen this before)
'stair's top pair turned left' is the wordplay.
'top' says to take the initial letters.
'turned' indicates an anagram.
'left' becomes 'l' (common abbreviation).
The initial letter of 'stairs' is 's'.
'pair' anagrammed gives 'pira'.
's'+'pira'+'l'='SPIRAL'
(Other definitions for spiral that I've seen before include "Shape of coil or helix" , "Whorl" , "Corkscrew shape (6)." , "Scandalise" , "design for some staircase" .)