A foreign assassin went ahead without scruples (12)
I believe the answer is:
unprincipled
'without scruples' is the definition.
(without moral scruples)
'a foreign assassin went ahead' is the wordplay.
'a foreign' becomes 'un' ('a' translated into French).
'assassin' becomes 'princip' (Gavrilo Princip shot Archduke Franz Ferdinand).
'went ahead' becomes 'led' (led the way).
'un'+'princip'+'led'='UNPRINCIPLED'
(Other definitions for unprincipled that I've seen before include "Lacking integrity" , "Without integrity" , "Immoral" , "Lacking moral scruples" , "Lacking morals" .)