Old marathoner finally entered and ran (6)
I believe the answer is:
darted
'ran' is the definition.
'old marathoner finally entered' is the wordplay.
'old' becomes 'dated' (I've seen this in other clues).
'marathoner finally' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'entered' is an insertion indicator.
'dated' enclosing 'r' is 'DARTED'.
'and' is the link.
(Other definitions for darted that I've seen before include "Dashed, flitted" , "Was fleet" , "Dashed on the Dublin transport system?" , "Moved suddenly like game missile" , "Sniped" .)