Clue: Vital chaps who help rulers (9)
Answer: COREGENTS
Explanation: Definition: who help rulers
A charade of CORE ('vital') plus GENTS ('chaps'); a REGENT might be someone who helps a king, perhaps during his minority (altbough "help" may not always be the right word), and COREGENTS would be a pair of them (probably for each to counter the ambitions of the other).