Siren's sound (4)
I believe the answer is:
'siren's sound' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for wail that I've seen before include "blub" , "Utter a long sad cry" , "Cry of lament" , "Cry of grief" , "Long cry, as of siren" .)