Ruling lines (9)
I believe the answer is:
dynasties
'lines' is the definition.
(dynasty is a kind of line)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for dynasties that I've seen before include "Successive lines of powerful people from the same families" , "Houses" , "Sequences of powerful people in the same families" , "Lines of hereditary rulers often families" .)