Succession by top parliamentarian (9)
I believe the answer is:
roundhead
'parliamentarian' is the definition.
(I know that parliamentarian can be written as Roundhead)
'succession by top' is the wordplay.
'succession by' becomes 'round' (I am not sure about this - if you are sure you should believe this answer much more).
'top' becomes 'head' (head is a kind of top).
'round'+'head'='ROUNDHEAD'
Can you help me to learn more?
(Other definitions for roundhead that I've seen before include "antimonarchist" , "Parliamentarian (hist.)" , "An enemy of Charles" , "Supporter of Parliament during the English Civil Wars" , "Old soldier" .)