Constant smoker seen in Vietnam (4)
I believe the answer is:
etna
'constant smoker' is the definition.
(I've seen this before)
'in vietnam' is the wordplay.
'in' says the answer is hidden in the clue.
'ETNA' is hidden within 'vietnam'.
'seen' acts as a link.
(Other definitions for etna that I've seen before include "Neat kind of European volcano" , "Volcanic mount in Sicily" , "Threatening mountain" , "Sicilian volcano" , "feature spewing lava" .)