Fervent low wail (4)
I believe the answer is:
keen
This clue could be a double definition.
'fervent' is the first definition.
(I've seen this before)
'low wail' is the second definition.
'keen' can be an answer for 'wail' (I've seen this in another clue). I am unsure of the 'low' bit.
Can you help me to learn more?
(Other definitions for keen that I've seen before include "Razor-edged" , "Acute" , "Quick" , "Eager, enthusiastic" , "Innumerable" .)