Mr Ed supporting son who's standing provokingly? (8-5)
I believe the answer is:
stalking-horse
'who's standing provokingly?' is the definition.
(person standing for election to provoke a contest)
'mr ed supporting son' is the wordplay.
'mr ed' becomes 'talking horse' (Mr Ed, talking horse from 60s TV show).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'son' becomes 's' (genealogical abbreviation for son).
'talkinghorse' after 's' is 'STALKING-HORSE'.
(Other definitions for stalking-horse that I've seen before include "Mobile hide for hunter - candidate testing the waters" , "Candidate put up to conceal the real threat" , "Pretext; candidate as ploy" , "Trial candidate" , "Political pawn" .)