left wing

Concise Oxford English Dictionary © 2008 Oxford University Press:
left wing

▶noun

• 1 the radical, reforming, or socialist section of a political party or system.

  ■ [with ref. to the National Assembly in France (1789–91), where the nobles sat to the president's right and the commons to the left.]

• 2 the left side of a sports team on the field or of an army.

– derivatives
left-winger noun.