Out, in again; that’s the course! (5)
I believe the answer is:
route
'the course' is the definition.
(both can mean a path)
'out in again' is the wordplay.
'in' indicates putting letters inside.
'again' becomes 're' (prefix meaning 'again').
'out' inserted within 're' is 'ROUTE'.
'that's' is the link.
(Other definitions for route that I've seen before include "Established line of trave" , "Path to follow" , "Direction of travel" , "Course followed" , "Course to destination" .)