Go off course entering Belgrade via Tehran (7)
I believe the answer is:
deviate
'go off course' is the definition.
(I've seen this before)
'entering belgrade via tehran' is the wordplay.
'entering' says the answer is hidden in the clue.
'DEVIATE' is hidden in the letters of 'belgrade via tehran'.
(Other definitions for deviate that I've seen before include "Depart from an established course" , "Turn aside from accepted way" , "Go off track" , "Change from a course or standard" , "Diverge (from a course)" .)