Immediately aware of dubious practice of succeeding voting system (6)
I believe the answer is:
pronto
'immediately' is the definition.
(pronto can mean at once or quickly)
'aware of dubious practice of succeeding voting system' is the wordplay.
'aware of dubious practice of' becomes 'onto' (if you're onto someone you know what they're up to).
'succeeding' is a charade indicator (letters next to each other) (succeeding can mean going after or following).
'voting system' becomes 'PR' (proportional representation).
'onto' after 'pr' is 'PRONTO'.
(Other definitions for pronto that I've seen before include "Quick as you can, amigo" , "Right now, immediately" , "Proton (anag.)" , "Quickly, without delay" , "Italian telephonic greeting" .)