Astounds Belfast with brags, crazy (12)
I believe the answer is:
flabbergasts
'astounds' is the definition.
(to flabbergast is to astonish or astound)
'belfast with brags crazy' is the wordplay.
'with' is a charade indicator (letters next to each other).
'crazy' is an anagram indicator.
'belfast' put after 'brags' is 'bragsbelfast'.
'bragsbelfast' is an anagram of 'FLABBERGASTS'.
(Other definitions for flabbergasts that I've seen before include "floors" , "staggers!" .)