Volcano in Vietnam (4)
I believe the answer is:
etna
'volcano' is the definition.
(Mount Etna)
'in vietnam' is the wordplay.
'in' says the answer is hidden in the clue.
'ETNA' is hidden in the letters of 'vietnam'.
(Other definitions for etna that I've seen before include "Volcano in Italy" , "Sicilian hothead?" , "Italian smoker" , "Long-term smoker" , "Constant smoker" .)