Off on a course (4)
I believe the answer is:
away
'off' is the definition.
(synonyms)
'on a course' is the wordplay.
'on' says to put letters next to each other.
'course' becomes 'way' (way is a kind of course).
'a'+'way' is 'AWAY'.
(Other definitions for away that I've seen before include "At a distance; match" , "Chelsea's game at Old Trafford is" , "Location of match played on opponent's ground" , "On holiday" , "Absent - abroad" .)