Cromwellian's plump leader (9)
I believe the answer is:
roundhead
'cromwellian's' is the definition.
(supporter of Oliver Cromwell)
'plump leader' is the wordplay.
'plump' becomes 'round' (I've seen this before).
'leader' becomes 'head' (head is a kind of leader).
'round'+'head'='ROUNDHEAD'
(Other definitions for roundhead that I've seen before include "antimonarchist" , "Old soldier" , "An enemy of Charles" , "Supporter of Parliament during the English Civil Wars" , "Cavalier's foe" .)