Capital backing in Vietnam, maybe (5)
I believe the answer is:
amman
'capital' is the definition.
(capital of Jordan)
'backing in vietnam maybe' is the wordplay.
'backing' says the letters should be written in reverse.
'in' indicates a hidden word.
'namma' is hidden in the letters of 'vietnam maybe'.
'namma' in reverse letter order is 'AMMAN'.
(Other definitions for amman that I've seen before include "Jordanian capital" , "Jordanian city" , "Middle East capital" , "City of Jordan" , "Asian capital" .)