Assassin popular in New Jersey area (5)
I believe the answer is:
ninja
'assassin' is the definition.
(I've seen this before)
'popular in new jersey area' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'in' is an insertion indicator.
'new jersey' becomes 'nj' (abbreviation).
'area' becomes 'a' (maths abbreviation).
'in' inserted inside 'nj' is 'ninj'.
'ninj'+'a'='NINJA'
(Other definitions for ninja that I've seen before include "Stealthy Japanese spy" , "Stealthy warrior" , "martial art expert" , "14th-century Japanese soldier of fortune" , "Assassin from Japan" .)