Lack of enthusiasm for one heading off around track (6)
I believe the answer is:
apathy
'lack of enthusiasm' is the definition.
(I've seen this before)
'for one heading off around track' is the wordplay.
'for one' becomes 'say' (both can mean 'for example').
'heading off' means to remove the first letter.
'around' indicates putting letters inside.
'track' becomes 'path' ('path' can be a synonym of 'track'**).
'say' with its initial letter removed is 'ay'.
'ay' going around 'path' is 'APATHY'.
(Other definitions for apathy that I've seen before include "Flatness" , "Lack of interest or enthusiasm" , "Absence of emotion or enthusiasm" , "lack of involvement" , "Impassivity - lack of feeling" .)