Acoustical instrument (5)
I believe the answer is:
siren
'acoustical instrument' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for siren that I've seen before include "Warning signal generator" , "Alarm - fascinating woman" , "Rinse (anag) - sea nymph" , "Loud whistle or sailors' enchantress" , "Bewitching singer" .)