Dangerous customer two idiots elected (8)
I believe the answer is:
assassin
'dangerous customer' is the definition.
(a dangerous person)
'two idiots elected' is the wordplay.
'two' shows that letters should be duplicated.
'idiots' becomes 'ass' (I can't explain this - if you can you should give a lot more credence to this answer).
'elected' becomes 'in' (in government).
'ass' doubled is 'assass'.
'assass'+'in'='ASSASSIN'
(Other definitions for assassin that I've seen before include "butcher" , "Hitman" , "One making hits" , "A killer of political or religious leader" , "Murderer (of prominent figure)" .)