One constituent is in bar supporting both parties (10)
I believe the answer is:
'supporting both parties' is the definition.
(supporting or involving multiple political parties)
'one constituent is in bar' is the wordplay.
'one' becomes 'I' (Roman numeral).
'constituent' becomes 'part' (I've seen this before).
'in' is an insertion indicator.
'bar' becomes 'ban' (I've seen this in another clue).
'ipartis' put into 'ban' is 'BIPARTISAN'.
(Other definitions for bipartisan that I've seen before include "Associated with two parties" , "With both sides' agreement" , "between political groups" , "using both sides" , "Cooperative" .)