Siren's obscure wind instrument (7)
I believe the answer is:
foghorn
'siren's' is the definition.
(I've seen this before)
'obscure wind instrument' is the wordplay.
'obscure' becomes 'fog' ('fog' can be a synonym of 'obscure').
'wind instrument' becomes 'horn' (horn is a kind of wind instrument).
'fog'+'horn'='FOGHORN'
(Other definitions for foghorn that I've seen before include "Ship's hooter" , "Ship-warning siren" , "Warning device" , "Ship warner" , "Warning for ships" .)