Mantel terribly keen (6)
I believe the answer is:
lament
'keen' is the definition.
(keen can mean to wail with grief)
'mantel terribly' is the wordplay.
'terribly' indicates anagramming the letters.
'mantel' with letters rearranged gives 'LAMENT'.
(Other definitions for lament that I've seen before include "Wail" , "requiem" , "Song expressing grief" , "Sad song" , "Expr. of grief; mourn" .)