Ancient supercontinent (7)
I believe the answer is:
pangaea
'ancient supercontinent' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pangaea that I've seen before include "Original supercontinent" , "Original large continent" , "ancient landmass" , "Hypothetical early continent, all earth in Greek" , "land now broken up" .)