Middle East national terrorists rejected intelligence measure (5)
I believe the answer is:
iraqi
'middle east national' is the definition.
(I've seen this before)
'terrorists rejected intelligence measure' is the wordplay.
'terrorists' becomes 'ira' (Irish Republican Army).
'rejected' says the letters should be written in reverse.
'intelligence measure' becomes 'IQ'.
'iq' written backwards gives 'qi'.
'ira'+'qi'='IRAQI'
(Other definitions for iraqi that I've seen before include "Baghdad-born?" , "Foreign citizen" , "Probably a Muslim" , "Native of Fallujah, perhaps" , "foreigner" .)