Those Corbynistas remaining? (4)
I believe the answer is:
left
'remaining?' is the definition.
('left' can be a synonym of 'remaining')
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for left that I've seen before include "Outstanding" , "not consumed" , "Punch, perhaps" , "Went; port" , "Departed or bequeathed" .)