Supplier of caviar, one who operates across heart of Vietnam (8)
I believe the answer is:
sturgeon
'supplier of caviar' is the definition.
'one who operates across heart of vietnam' is the wordplay.
'one who operates' becomes 'surgeon' (I've seen this before).
'across' indicates putting letters inside.
'heart of' indicates the central letters.
The centre of 'vietnam' is 't'.
'surgeon' going around 't' is 'STURGEON'.
(Other definitions for sturgeon that I've seen before include "Fish producing caviare" , "Something fishy here" , "Source of caviar" , "Fish from which caviar is obtained" , "Our gents caught a large fish" .)