Clue: Holy man supporting Pope Romanus's cardinals, that is (6)
Answer: PRIEST
Explanation: Definition: Holy man
A charade of PR ('**P**ope **R**omanus's cardinals') plus I.E. (*Is Id Est*, 'that is') plus ST (saint, 'holy man') with an extended definition. Pope Romanus? Sorry, you missed him. He was Pope from August to November 897.