Awful rejection of wedding vow by Oscar from America (6)
I believe the answer is:
odious
'awful' is the definition.
(thesaurus)
'rejection of wedding vow by oscar from america' is the wordplay.
'rejection of' is a reversal indicator.
'wedding vow' becomes 'ido' ('I do' is part of the traditional wedding vows).
'by' says to put letters next to each other.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'from america' becomes 'us'.
'ido' written backwards gives 'odi'.
'odi'+'o'+'us'='ODIOUS'
(Other definitions for odious that I've seen before include "Really obnoxious, generally disliked" , "Extremely unpleasant" , "Detestable, execrable" , "Comparisons are said to be thus" , "Hateful and disgusting" .)