Run one marathon, perhaps (4)
I believe the answer is:
race
'marathon perhaps' is the definition.
(marathon is a kind of race)
'run one' is the wordplay.
'run' becomes 'r' (cricket abbreviation).
'one' becomes 'ace' (ace has a value of one in some card games).
'r'+'ace'='RACE'
(Other definitions for race that I've seen before include "See 22" , "People; dash" , "compete to come first" , "eg The Oaks" , "The ... is not to the swift (Bible)" .)