Bring a charge against (9)
(Other definitions for prosecute that I've seen before include "Represent the Crown in court" , "Pursue" , "Take legal action" , "Carry out assessment" , "Bring a criminal action against" .)
'bring a charge against' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for impeach that I've seen before include "Bring an accusation against" , "Charge with a crime (against the state)" , "Charge a public official with an offence" , "In America, charge with misconduct - say President" , "Charge with treason, etc" .)