Big electoral victory is ground-breaking (9)
I believe the answer is:
landslide
'big electoral victory' is the definition.
'landslide' can be an answer for 'victory' (I've seen this before). I am unsure of the 'big electoral' bit.
'ground-breaking' is the wordplay.
'ground' becomes 'land' (synonyms).
'breaking' becomes 'slide' (I can't justify this - if you can you should believe this answer much more).
'land'+'slide'='LANDSLIDE'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for landslide that I've seen before include "convincing victory" , "significant win in contest" , "big victory" , "Runaway" , "Overwhelming election majority" .)