River with a vast delta in Vietnam (6)
I believe the answer is:
mekong
'river with a vast delta in vietnam' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for mekong that I've seen before include "River and delta in southern Vietnam" , "Asian banker" , "Asian river, flows into South China Sea" , "River with vast delta in Vietnam" , "River of southeast Asia" .)