The Collaborative International Dictionary of English v.0.48:
backbencher \backbencher\ (b[a^]k"b[e^]nch*[~e]r), n. a member of the House of Commons of Great Britain who is not a party leader. [WordNet 1.5]WordNet (r) 3.0 (2006):
backbencher n 1: a member of the House of Commons who is not a party leader