Terrible setting for end of boring requiem (5)
I believe the answer is:
dirge
'requiem' is the definition.
(synonyms)
'terrible setting for end of boring' is the wordplay.
'terrible' becomes 'dire' (synonyms).
'setting for' indicates putting letters inside.
'end of' says to take the final letters.
The last letter of 'boring' is 'g'.
'dire' placed around 'g' is 'DIRGE'.
(Other definitions for dirge that I've seen before include "Keen; lament" , "A depressing number" , "Song or hymn of mourning" , "Sombre music" , "No joy in this music" .)