Secure team bagging 50 in big victory (9)
I believe the answer is:
landslide
'big victory' is the definition.
(decisive electoral victory)
'secure team bagging 50' is the wordplay.
'secure' becomes 'land' (I've seen this in another clue).
'team' becomes 'side' (both can mean a sports team).
'bagging' indicates putting letters inside (I've seen this in other clues).
'50' becomes 'l' (Roman numeral).
'side' placed around 'l' is 'slide'.
'land'+'slide'='LANDSLIDE'
'in' acts as a link.
(Other definitions for landslide that I've seen before include "easy victory" , "Massive election win" , "Overwhelming success" , "Movement at grass-roots level" , "big victory" .)