Support for progressives' leader (4)
I believe the answer is:
prop
'support' is the definition.
(prop is a kind of support)
'for progressives' leader' is the wordplay.
'for' becomes 'pro' (I've seen this before).
'leader' says to take the initial letters.
The initial letter of 'progressives' is 'p'.
'pro'+'p'='PROP'
(Other definitions for prop that I've seen before include "What actor may carry" , "Portable film set item" , "A rigid supprot or piece of satge furniture (4)" , "One is usually among the strongest and heaviest rugby players" , "Support; rugby forward" .)