Abhorrent national song about American adult (8)
I believe the answer is:
anathema
'abhorrent' is the definition.
(I've seen this before)
'national song about american adult' is the wordplay.
'national song' becomes 'anthem' (I've seen this before).
'about' indicates putting letters inside.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'adult' becomes 'a' (abbreviation).
'anthem' going around 'a' is 'anathem'.
'anathem'+'a'='ANATHEMA'
(Other definitions for anathema that I've seen before include "Bete noir" , "Curse; object of loathing" , "hated person" , "Object of detestation" , "Person or thing that's abhorred" .)