Assassin infiltrating town in Japan (5)
I believe the answer is:
ninja
'assassin' is the definition.
(I've seen this before)
'infiltrating town in japan' is the wordplay.
'infiltrating' indicates a hidden word.
'NINJA' is hidden within 'town in japan'.
(Other definitions for ninja that I've seen before include "Assassin from Japan" , "Assassins" , "Japanese trained assassin" , "Teenage Mutant ... Turtle (toy)" , "Animated turtle" .)