Experts holding up Republican contests (5)
I believe the answer is:
races
'contests' is the definition.
(I know this)
'experts holding up republican' is the wordplay.
'experts' becomes 'aces' (ace is a kind of expert).
'holding up' says to put letters next to each other (some letters go underneath or hold up others).
'republican' becomes 'R' (political abbreviation).
'aces' put after 'r' is 'RACES'.
(Other definitions for races that I've seen before include "Moves very fast" , "Speed competitions for various peoples" , "Runs" , "Speed contests for horses, cars etc" , "Sprint and marathon?" .)