Manage to compete in marathon? (3)
I believe the answer is:
run
This clue is a triple definition!
'manage' is the first definition.
(both can mean to organise)
'to compete' is the second definition.
(running is a kind of competing)
'in' is a link.
'marathon?' is the third definition.
(marathon is a kind of run)
(Other definitions for run that I've seen before include "Possibly an extra" , "Jog; manage" , "A single" , "Dart" , "publish" .)