Curse a boring song, Sinatra's finale (8)
I believe the answer is:
anathema
'curse' is the definition.
(anathema is a kind of curse)
'a boring song sinatra's finale' is the wordplay.
'boring' indicates putting letters inside (some letters must 'bore' a hole into other letters).
'song' becomes 'anthem' (anthem is a kind of song**).
'finale' suggests the final letters.
The last letter of 'sinatra' is 'a'.
'a' going into 'anthem' is 'anathem'.
'anathem'+'a'='ANATHEMA'
(Other definitions for anathema that I've seen before include "Abomination, bugbear" , "Bete noire" , "Accursed by the Church" , "One detested, cursed" , "Something hated" .)