American rewrites anthem - it's a thing of loathing (8)
I believe the answer is:
Here is my best explanation:
'thing of loathing' is the definition.
'anathema' can be an answer for 'thing' (I have seen 'hateful thing' mean 'anathema' so perhaps 'thing' could also mean 'anathema'). I am unsure of the 'of loathing' bit.
'american rewrites anthem it's a' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'rewrites' is an anagram indicator.
'anthem' anagrammed gives 'nathem'.
(Other definitions for anathema that I've seen before include "Bete noir", "Accursed by the Church", "One hated", "Object of detestation", "denunciation", "Condemned or excommunicated by church", "target of hate", "odious", "Object of repulsion" and "thing we detest".)