Killer popular with two idiots (8)
I believe the answer is:
assassin
'killer' is the definition.
(I know that assassin is a type of killer)
'popular with two idiots' is the wordplay.
'two idiots' can mean two replacements for 'idiot'.
'popular' becomes 'in' ('in' can mean trendy).
'with' says to put letters next to each other.
'idiot' becomes 'ass' (both can mean a stupid person).
'idiot' becomes 'ass' (both can mean a stupid person).
'ass'+'ass'='assass'
'in' put after 'assass' is 'ASSASSIN'.
(Other definitions for assassin that I've seen before include "Bravo!" , "Asians SS? (Anag)" , "Political murderer" , "Murderer, killer" , "Killer of prominent person" .)