Weather indicator sounds useless (4)
I believe the answer is:
vane
'weather indicator' is the definition.
(I've seen this in another clue)
'sounds useless' is the wordplay.
'sounds' shows a homophone (sound like).
'useless' becomes 'vain' (I've seen this in another clue).
'vain' is a homophone of 'VANE'.
(Other definitions for vane that I've seen before include "Weather-cock sounds full of itself" , "Weather blade" , "Blade of windmill" , "The weather-cock sounds conceited" , "Wind pointer" .)