Junior reporter on a Caribbean island (4)
I believe the answer is:
cuba
'caribbean island' is the definition.
(I've seen this in another clue)
'junior reporter on a' is the wordplay.
'junior reporter' becomes 'cub' (I've seen this in another clue).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'cub'+'a'='CUBA'
(Other definitions for cuba that I've seen before include "tropical country" , "Its capital is Havana" , "Largest West Indies island" , "Caribbean island republic" , "neighbour to America" .)