Vote Labour rejected by chance (6)
I believe the answer is:
ballot
'vote' is the definition.
(synonyms)
'labour rejected by chance' is the wordplay.
'labour' becomes 'lab' (abbreviation).
'rejected' says the letters should be written in reverse.
'by' means one lot of letters go next to another.
'chance' becomes 'lot' (as in drawing by lot).
'lab' in reverse letter order is 'bal'.
'bal'+'lot'='BALLOT'
(Other definitions for ballot that I've seen before include "Referendum" , "Plebiscite" , "Voting procedure" , "Lottery (eg, for tickets)" , "System of voting secretly" .)