Overwhelming victory for transparency by nation (9)
I believe the answer is:
landslide
'overwhelming victory' is the definition.
(I've seen this before)
'transparency by nation' is the wordplay.
'transparency' becomes 'slide' (slide is a kind of transparency).
'by' is a charade indicator (letters next to each other).
'nation' becomes 'land' ('land' can be a synonym of 'nation'**).
'slide' after 'land' is 'LANDSLIDE'.
'for' acts as a link.
(Other definitions for landslide that I've seen before include "Massive election win" , "big victory" , "big majority" , "Movement at grass-roots level" , "significant win in contest" .)