Test of French in viva-voce (6)
I believe the answer is:
ordeal
'test' is the definition.
(thesaurus)
'of french in viva-voce' is the wordplay.
'of french' becomes 'de'.
'in' is an insertion indicator.
'viva voce' becomes 'oral' ('oral' can be a synonym of 'viva voce').
'de' inserted within 'oral' is 'ORDEAL'.
(Other definitions for ordeal that I've seen before include "Suffering - trial" , "Difficult experience, a tough time" , "Long painful experience" , "Unpleasant and prolonged experience" , "Severly trying experience" .)