Runs against one in contest (4)
I believe the answer is:
race
'contest' is the definition.
(race is a kind of contest)
'runs against one' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'against' is a charade indicator (letters next to each other).
'one' becomes 'ace' ('ace' can be a synonym of 'one').
'r'+'ace'='RACE'
'in' acts as a link.
(Other definitions for race that I've seen before include "descent" , "Breed" , "Run, dash" , "People; dash" , "Career - people" .)