Labour's reverse of fortune enthralling one (4)
I believe the answer is:
toil
'labour's' is the definition.
('toil' can be a synonym of 'labour')
'reverse of fortune enthralling one' is the wordplay.
'reverse of' says the letters should be written in reverse.
'fortune' becomes 'lot' ('lot' can be a synonym of 'fortune').
'enthralling' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'lot' placed around 'i' is 'liot'.
'liot' back-to-front is 'TOIL'.
(Other definitions for toil that I've seen before include "Work hard and long" , "Hard physical work" , "Work really hard" , "Manual labour" , "Drudgery" .)