Masked assassin is somewhat forlorn in jail (5)
I believe the answer is:
ninja
'masked assassin' is the definition.
(I've seen this in another clue)
'somewhat forlorn in jail' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'NINJA' can be found hidden inside 'forlorn in jail'.
'is' is the link.
(Other definitions for ninja that I've seen before include "Mercenary in feudal Japan" , "Teenage Mutant ... Turtle (toy)" , "Stealthy Japanese fighter" , "Expert in espionage" , "Stealthy Japanese spy" .)