Murder double idiot with one neat twist (11)
I believe the answer is:
assassinate
'murder' is the definition.
(I've seen this before)
'double idiot with one neat twist' is the wordplay.
'double' shows that letters should be duplicated.
'idiot' becomes 'ass' (I've seen this before).
'with' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'twist' is an anagram indicator (a twisted or distorted version of the letters).
'ass' doubled is 'assass'.
'neat' is an anagram of 'nate'.
'i' inserted inside 'nate' is 'inate'.
'assass'+'inate'='ASSASSINATE'
(Other definitions for assassinate that I've seen before include "Kill for a cause" , "Murder (a public figure)" , "Kill (a VIP)" , "Murder (a political or religious leader)" , "top" .)