Dumbfounded by a dishonest arrangement (10)
I believe the answer is:
astonished
'dumbfounded' is the definition.
('astonished' can be a synonym of 'dumbfounded')
'by a dishonest arrangement' is the wordplay.
'by' means one lot of letters go next to another.
'arrangement' is an anagram indicator.
'dishonest' is an anagram of 'stonished'.
'a' put next to 'stonished' is 'ASTONISHED'.
(Other definitions for astonished that I've seen before include "Sad one, this (anag)" , "given a bombshell?" , "Full of amazement" , "Greatly surprised" , "dumbfounded" .)