Outsider in marathon attempt (4,4)
I believe the answer is:
long shot
'outsider' is the definition.
(I've seen this before)
'marathon attempt' is the wordplay.
'marathon' becomes 'long' (marathon can mean long or lengthy).
'attempt' becomes 'shot' (shot is a kind of attempt).
'long'+'shot'='LONG SHOT'
'in' is the link.
(Other definitions for long shot that I've seen before include "bet that could bring big return" , "telephoto picture?" , "An undertaking with little chance of success" , "Attempt not expected to succeed" , "Wild venture; distant film view" .)