Support for the party leader (4)
I believe the answer is:
prop
'support' is the definition.
(prop up can mean to support)
'for the party leader' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'the party leader' becomes 'p' (leader of the word 'party').
'pro'+'p'='PROP'
(Other definitions for prop that I've seen before include "Support to keep from falling" , "Support - item on stage" , "player involved in scrum" , "Temporary support" , "A rigid supprot or piece of satge furniture (4)" .)