Jack held hostage by girl assassin (5)
I believe the answer is:
ninja
'assassin' is the definition.
(I've seen this before)
'jack held hostage by girl' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'held hostage by' indicates putting letters inside (inserted letters are held hostage).
'girl' becomes 'nina' ().
'j' inserted into 'nina' is 'NINJA'.
(Other definitions for ninja that I've seen before include "Deadly type" , "old mercenary" , "Japanese expert in stealth and martial arts" , "Japanese spy or assassin" , "Assassins" .)