Underdeveloped countries (5,5)
'underdeveloped countries' is the definition.
(I've seen this before)
This is all the clue.
'underdeveloped countries' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for third world that I've seen before include "Developing countries" , "future kind of war" , "developing nations" , "Poorer nations" , "Developing economies" .)