It shows wind direction (4)
I believe the answer is:
vane
'it shows wind direction' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for vane that I've seen before include "Part of a weathercock" , "The weather-cock sounds conceited" , "Fin attached to the tail of an arrow" , "Device that rotates to show the direction of the wind" , "Windmill sail in 30 across" .)