Vote upsetting to Liberal and another party (6)
I believe the answer is:
ballot
'vote' is the definition.
(synonyms)
'upsetting to liberal and another party' is the wordplay.
'upsetting' is a reversal indicator.
'liberal' becomes 'l' (abbreviation).
'and' means one lot of letters go next to another.
'another party' becomes 'lab' (abbreviation for Labour. I am not sure about the 'another' bit.).
'to'+'l'+'lab'='tollab'
'tollab' in reverse letter order is 'BALLOT'.
(Other definitions for ballot that I've seen before include "Lottery to allocate (tickets) among applicants" , "Document listing voting alternatives" , "Election" , "Secret vote" , "System of voting secretly" .)