Parliamentarian leader holding up circular (9)
I believe the answer is:
roundhead
'parliamentarian' is the definition.
(I've seen this before)
'leader holding up circular' is the wordplay.
'leader' becomes 'head' (head is a kind of leader).
'holding up' means one lot of letters go next to another (some letters go underneath or hold up others).
'circular' becomes 'round' (synonyms).
'head' put after 'round' is 'ROUNDHEAD'.
(Other definitions for roundhead that I've seen before include "Parliamentarian (hist.)" , "Supporter of Parliament during the English Civil Wars" , "An enemy of Charles" , "Cavalier's foe" , "antimonarchist" .)