International organisation in illustration of old city (5)
I believe the answer is:
punic
'of old city' is the definition.
(Punic means relating to Carthage)
'international organisation in illustration' is the wordplay.
'international organisation' becomes 'un'.
'in' indicates putting letters inside.
'illustration' becomes 'pic' (short for picture).
'un' placed into 'pic' is 'PUNIC'.
(Other definitions for punic that I've seen before include "Carthaginian" , "ancient language" , "Relating to Carthage" .)