Evershed, who left New Star in September 2008, is claiming constructive dismissal under the whistleblower act.
Evershed said he had earlier been told he could not pursue the claim because it had been filed incorrectly and had overrun a time limit.
He has now won an appeal against that decision and could claim up to £1m, according to reports.
Speaking to the Guardian, Evershed said: “I have complained about the way Duffield and his senior people bullied. It is almost the story of how New Star collapsed. Duffield made life impossible for us.”
Evershed now works at Hargreave Hale. He left New Star in September 2008 after six years with the firm. Evershed managed the £60m select opportunities fund since its launch in May 2002.