Gadget atop a barn (4)
I believe the answer is:
vane
'gadget atop a barn' 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 "Indicator of wind direction" , "Blade of windmill" , "Windmill sail in 30 across" , "Wind pointer" , "Fin attached to the tail of an arrow" .)