Challenging Turkish official isn't unusual (7)
I believe the answer is:
against
'challenging' is the definition.
(eg opposing in a battle)
'turkish official isn't unusual' is the wordplay.
'turkish official' becomes 'aga' (military commander eg in the Ottoman Empire).
'unusual' indicates anagramming the letters.
'isnt' anagrammed gives 'inst'.
'aga'+'inst'='AGAINST'
(Other definitions for against that I've seen before include "In conflict with" , "Not in favour of" , "In competition with" , "Facing" , "Anti, touching" .)