Wind indicator is useless, we hear (4)
I believe the answer is:
vane
'wind indicator' is the definition.
(I've seen this in another clue)
'useless we hear' is the wordplay.
'useless' becomes 'vain' (I've seen this before).
'we hear' shows a homophone (sound like).
'vain' sounds like 'VANE'.
'is' acts as a link.
(Other definitions for vane that I've seen before include "Weather-cock sounds full of itself" , "Indicator of wind direction" , "Blade of windmill" , "The weather-cock sounds conceited" , "Fin attached to the tail of an arrow" .)