Immediately suspicious of supporting voting system (6)
I believe the answer is:
pronto
'immediately' is the definition.
(thesaurus)
'suspicious of supporting voting system' is the wordplay.
'suspicious of' becomes 'onto' (as in 'the police are onto us').
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'voting system' becomes 'pr' (I've seen this in other clues).
'onto' put after 'pr' is 'PRONTO'.
(Other definitions for pronto that I've seen before include "Double quick!" , "Quick as you can, amigo" , "Proton (anag.)" , "Right away, quickly" , "Quickly (sl.)" .)