Parliamentarian leader following prescribed course (9)
I believe the answer is:
roundhead
'parliamentarian' is the definition.
(I've seen this before)
'leader following prescribed course' is the wordplay.
'leader' becomes 'head' (head is a kind of leader).
'following' is a charade indicator (letters next to each other).
'prescribed course' becomes 'round' (a course or route, as in doing one's rounds).
'head' after 'round' is 'ROUNDHEAD'.
(Other definitions for roundhead that I've seen before include "Supporter of Parliament during the English Civil Wars" , "Parliamentarian (hist.)" , "Cavalier's foe" , "Old soldier" , "antimonarchist" .)