Parliamentarian drinks with leader (9)
I believe the answer is:
roundhead
'parliamentarian' is the definition.
(I know that parliamentarian can be written as Roundhead)
'drinks with leader' is the wordplay.
'drinks' becomes 'round' (as in a round of drinks).
'with' means one lot of letters go next to another.
'leader' becomes 'head' (head is a kind of leader).
'round'+'head'='ROUNDHEAD'
(Other definitions for roundhead that I've seen before include "Parliamentarian (hist.)" , "Supporter of Parliament during the English Civil Wars" , "Old soldier" , "An enemy of Charles" , "antimonarchist" .)