Ardent lament (4)
I believe the answer is:
keen
I believe this clue is a double definition.
'ardent' is the first definition.
(thesaurus)
'lament' is the second definition.
(keen is a kind of lament)
(Other definitions for keen that I've seen before include "display grief" , "Eager; dirge" , "Sharp - avid" , "Eager - to lament the dead" , "Enthusiastic or sharp" .)