Chain on the Mid-Atlantic Ridge (6)
I believe the answer is:
azores
'chain on the mid-atlantic ridge' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for azores that I've seen before include "North Atlantic islands, part of Portugal" , "Port. Atlantic islands" , "Atlantic island group" , "Group of Portuguese volcanic islands in N Atlantic" , "Group of North Atlantic volcanic islands" .)