Honourable to support revolutionary in the long run (8)
I believe the answer is:
marathon
'the long run' is the definition.
(long-distance run)
'honourable to support revolutionary' is the wordplay.
'honourable' becomes 'hon' (as in Rt Hon, Right Honourable).
'to support' means one lot of letters go next to another (in a down clue, some letters hold others up).
'revolutionary' becomes 'marat' (French revolutionary Jean-Paul Marat).
'hon' after 'marat' is 'MARATHON'.
'in' acts as a link.
(Other definitions for marathon that I've seen before include "Severe test of endurance" , "The longest foot race in sport" , "Long distance race" , "Pheidippides' starting point" , "Footrace of approx 26 miles" .)