Ambassador's sister vicious at heart (6)
I believe the answer is:
nuncio
'ambassador's' is the definition.
(I've seen this before)
'sister vicious at heart' is the wordplay.
'sister' becomes 'nun' (sister is a term of address for a nun).
'at heart' says to take the centre.
The centre of 'vicious' is 'cio'.
'nun'+'cio'='NUNCIO'
(Other definitions for nuncio that I've seen before include "Papal emissary" , "Papal ambassador" , "Pope's diplomatic representative" , "Sent by the Pope" , "Pope's ambassador" .)