A marathon in one direction (5)
I believe the answer is:
along
'in one direction' is the definition.
(move along)
'a marathon' is the wordplay.
'marathon' becomes 'long' (marathon can mean long or lengthy).
'a'+'long'='ALONG'
(Other definitions for along that I've seen before include "Moving in a constant direction on (something)" , "Beside - as groups often sing!" , "For the length of" , "Sea monster, opposite Charybdis" , "Lengthwise" .)