Useless device supposed to show wind direction, we’re told (4)
I believe the answer is:
vain
'useless' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for vain that I've seen before include "Conceited - or hopeless" , "Conceited and ineffectual" , "Conceited - futile" , "Concieted or pointless" , "Conceited, puffed-up" .)