Feature of Sicily or part of Vietnam (4)
I believe the answer is:
etna
'feature of sicily' is the definition.
(I've seen this before)
'part of vietnam' is the wordplay.
'part of' indicates a hidden word.
'ETNA' can be found hidden inside 'vietnam'.
'or' is the link.
(Other definitions for etna that I've seen before include "Neat volcano overturned" , "Volcano in Italy" , "Highest peak in Italy south of the Alps" , "Long-term smoker" , "burning mountain" .)