Spoil American for nothing with an addition to cake (8)
I believe the answer is:
marzipan
'addition to cake' is the definition.
(marzipan might decorate a cake)
'spoil american for nothing with an' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'american for nothing' becomes 'zip' (zip can mean nothing in US English).
'with' means one lot of letters go next to another.
'mar'+'zip'+'an'='MARZIPAN'
(Other definitions for marzipan that I've seen before include "almond concoction" , "Sweet almond paste" , "Almond-based cake covering" , "that's sweet" , "Cake icing made of ground almonds and sugar" .)