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'm not certain 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" .)