Definition of ultra-left in US English:

ultra-left

noun

the ultra-left
  • The extreme left wing of a political group or of the political spectrum.

    ‘it's true that the ultra-left of the party is grumbling over his selection’

adjective

  • Belonging to or representing the views of the extreme left wing of a political group or of the political spectrum.

    ‘the group's tactics appear to be inspired by ultra-left militants in Europe’