Sustenance rejected by American idiot (6)
I believe the answer is:
doofus
'idiot' is the definition.
(both can mean a foolish person)
'sustenance rejected by american' is the wordplay.
'sustenance' becomes 'food' (I've seen this before).
'rejected' is a reversal indicator.
'by' means one lot of letters go next to another.
'american' becomes 'US' (United States).
'food' back-to-front is 'doof'.
'doof'+'us'='DOOFUS'
(Other definitions for doofus that I've seen before include "American fool - US food (anag)" , "Nitwit" .)