your dict. is right - the fascist party was organized in local sections; all the sections of a "provincia" (then , the main administrative unit in Italy) were united into a "federazione"; and the big boss was the "segretario federale" - or, to keep it short, "il federale".

I think you you may safely skip "provincial" here and say "they killed the party Secretary of Milan", or something like that; you may also consider "the head" or even "the boss of the party" - depending on the speaker, of course; and hoping that by this point in the movie the audience will have realized that the party in question is the fascist one....