Complex sentence: Less well-regarded by colleagues was his friendly relations with Robert Maxwell, then the owner of the Daily Mirror, for whom Callan conducted celebrity interviews, but who was later implicated in large scale theft from his own employees' pension fund.
Split into simpler sentences: Less well-regarded by colleagues was his friendly relations with Robert Maxwell, then the owner of the Daily Mirror, for whom Callan conducted celebrity interviews. Maxwell was later implicated in large scale theft from his own employees' pension fund.