Red route south from Leeds seen in advance (6)
I believe the answer is:
commie
'red' is the definition.
(I've seen this before)
'route south from leeds seen in advance' is the wordplay.
'route south from leeds' becomes 'MI' (motorway from Leeds to London).
'seen in' indicates putting letters inside.
'advance' becomes 'come' (I've seen this in other clues).
'mi' placed into 'come' is 'COMMIE'.
(Other definitions for commie that I've seen before include "Eg, Marxist (colloq.)" , "Extreme left-winger" , "Red" , "Left wing" .)