Revolutionary darling, in the long run (8)
I believe the answer is:
marathon
'the long run' is the definition.
(long-distance run)
'revolutionary darling' is the wordplay.
'revolutionary' becomes 'marat' (French revolutionary Jean-Paul Marat).
'darling' becomes 'hon' (I've seen this in another clue).
'marat'+'hon'='MARATHON'
'in' is the link.
(Other definitions for marathon that I've seen before include "Long run (with a wall to get through!)" , "Long and arduous" , "Severe test of endurance" , "Long distance race" , "Footrace of approx 26 miles" .)