Most trustworthy party behind South Africa (6)
I believe the answer is:
safest
'most trustworthy' is the definition.
(I've seen this in another clue)
'party behind south africa' is the wordplay.
'party' becomes 'fest'.
'behind' means one lot of letters go next to another.
'south africa' becomes 'SA' (abbreviation).
'fest' put after 'sa' is 'SAFEST'.
(Other definitions for safest that I've seen before include "Least risky" , "Most secure and protected" , "Most dependable" , "most reliable" , "Freest from risk" .)