Barn topper (4)
I believe the answer is:
'barn topper' 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" , "Part of a weathercock" , "Windmill sail in 30 across" , "The weather-cock sounds conceited" , "Fin attached to the tail of an arrow" .)