Jack in girl, trained assassin (5)
I believe the answer is:
ninja
'trained assassin' is the definition.
(I've seen this before)
'jack in girl' is the wordplay.
'jack' becomes 'j' (playing card abbreviation).
'in' means one lot of letters goes inside another.
'girl' becomes 'nina' (**).
'j' inserted inside 'nina' is 'NINJA'.
(Other definitions for ninja that I've seen before include "Japanese mercenary warrior" , "Japanese spy or assassin" , "Japanese martial arts mercenary" , "Practioner of ninjitsu" , "old mercenary" .)