Barn roof spinner (4)
I believe the answer is:
vane
'barn roof spinner' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for vane that I've seen before include "Device that rotates to show the direction of the wind" , "Weather blade" , "It shows wind direction" , "Blade of a propeller" , "Part of a weathercock" .)