Last to sing in dreadful lament (5)
I believe the answer is:
dirge
'lament' is the definition.
(synonyms)
'last to sing in dreadful' is the wordplay.
'last to' says to take the final letters.
'in' indicates putting letters inside.
'dreadful' becomes 'dire' (synonyms).
The last letter of 'sing' is 'g'.
'g' placed into 'dire' is 'DIRGE'.
(Other definitions for dirge that I've seen before include "Melancholy song" , "Sombre music" , "Funeral lament" , "lamentable composition" , "dreary tune" .)