Trained assassin captured by American in Jamaica (5)
I believe the answer is:
ninja
'trained assassin' is the definition.
(I've seen this before)
'captured by american in jamaica' is the wordplay.
'captured by' indicates the answer is hidden within the clue.
'NINJA' is hidden within 'american in jamaica'.
(Other definitions for ninja that I've seen before include "Expert in espionage" , "Japanese mercenary warrior" , "Feudal assassin" , "Practioner of ninjitsu" , "Teenage Mutant ... Turtle (toy)" .)