Clue: Olympic organisation welcomes reintroduction of popular Greek dialect (5)
Answer: IONIC
Explanation: Definition: Greek dialect 

**IOC** (International Olympic Committee, so "Olympic organisation") *welcomes* [*reintroduction of*] **<=IN** ("popular")
 

 
*I assume that the setter intends reintroduction to mean "introducing back", but that's a set too far for me.