Piercing wail (4)
I believe the answer is:
keen
I believe this is a double definition.
'piercing' is the first definition.
(synonyms)
'wail' is the second definition.
(I've seen this in another clue)
(Other definitions for keen that I've seen before include "Diligent" , "Piercing" , "Sharp - lament" , "express sorrow" , "Penetrating; ardent" .)