Extremist in Istanbul, trapped (5)
I believe the answer is:
ultra
'extremist' is the definition.
('ultra' can be a synonym of 'extremist')
'in istanbul trapped' is the wordplay.
'in' says the answer is hidden in the clue.
'ULTRA' can be found hidden inside 'istanbul trapped'.
(Other definitions for ultra that I've seen before include "Above, beyond, super" , "Extreme - extremist" , "extraordinary" , "Political extremist" , "Secret intelligence" .)