One blocking revolutionary group in Labour (4)
I believe the answer is:
toil
'labour' is the definition.
('toil' can be a synonym of 'labour')
'one blocking revolutionary group' is the wordplay.
'one' becomes 'i' (Roman numeral).
'blocking' means one lot of letters goes inside another.
'revolutionary' says the letters should be written in reverse.
'group' becomes 'lot' (lot is a kind of group**).
'lot' written backwards gives 'tol'.
'i' put into 'tol' is 'TOIL'.
'in' is the link.
(Other definitions for toil that I've seen before include "Manual labour" , "Work really hard" , "Work hard and long" , "Hard physical work" , "Work extremely hard" .)