Assign assistant to go over to Post Office in Turkish capital (7)
I believe the answer is:
appoint
'assign' is the definition.
(assign can mean to appoint or delegate)
'assistant to go over to post office in turkish capital' is the wordplay.
'assistant' becomes 'PA' (personal assistant).
'to go over' shows that the letters should be reversed in order.
'to post office' becomes 'po' (abbreviation).
'capital' says to take the initial letters.
The initial letter of 'turkish' is 't'.
'pa' reversed gives 'ap'.
'ap'+'po'+'in'+'t'='APPOINT'
(Other definitions for appoint that I've seen before include "Name" , "get the post sorted" , "Pick" , "Ordain" , "Assign job to (someone)" .)