The Collaborative International Dictionary of English v.0.48:
muckraker \muck"rak`er\ (m[u^]k"r[=a]k`[~e]r), n. A person who habitually muckrakes. [PJC]WordNet (r) 3.0 (2006):
muckraker n 1: one who spreads real or alleged scandal about another (usually for political advantage) [syn: muckraker, mudslinger]